Addressed Yang's comments, I have removed usage of const throughout for consistency

include/grpcpp/security/credentials_impl.h

@@ -254,7 +254,7 @@ class MetadataCredentialsPlugin {
       const grpc::AuthContext& channel_auth_context,
       std::multimap<grpc::string, grpc::string>* metadata) = 0;
 
-  virtual grpc::string DebugString() const {
+  virtual grpc::string DebugString() {
     return "MetadataCredentialsPlugin did not provide a debug string";
   }
 };

src/core/lib/security/credentials/composite/composite_credentials.cc

@@ -119,7 +119,7 @@ void grpc_composite_call_credentials::cancel_get_request_metadata(
   GRPC_ERROR_UNREF(error);
 }
 
-std::string grpc_composite_call_credentials::debug_string() const {
+std::string grpc_composite_call_credentials::debug_string() {
   std::vector<std::string> outputs;
   for (auto& inner_cred : inner_) {
     outputs.emplace_back(inner_cred->debug_string());

src/core/lib/security/credentials/composite/composite_credentials.h

@@ -21,7 +21,7 @@
 
 #include <grpc/support/port_platform.h>
 
-#include <string.h>
+#include <string>
 
 #include "src/core/lib/gprpp/inlined_vector.h"
 #include "src/core/lib/gprpp/ref_counted_ptr.h"
@@ -93,7 +93,7 @@ class grpc_composite_call_credentials : public grpc_call_credentials {
   }
 
   const CallCredentialsList& inner() const { return inner_; }
-  std::string debug_string() const override;
+  std::string debug_string() override;
 
  private:
   void push_to_inner(grpc_core::RefCountedPtr<grpc_call_credentials> creds,

src/core/lib/security/credentials/credentials.h

@@ -254,7 +254,7 @@ struct grpc_call_credentials
     return min_security_level_;
   }
 
-  virtual std::string debug_string() const {
+  virtual std::string debug_string() {
     return "grpc_call_credentials did not provide debug string";
   }
 

src/core/lib/security/credentials/fake/fake_credentials.h

@@ -77,9 +77,7 @@ class grpc_md_only_test_credentials : public grpc_call_credentials {
   void cancel_get_request_metadata(grpc_credentials_mdelem_array* md_array,
                                    grpc_error* error) override;
 
-  std::string debug_string() const override {
-    return "MD only Test Credentials";
-  };
+  std::string debug_string() override { return "MD only Test Credentials"; };
 
  private:
   grpc_mdelem md_;

src/core/lib/security/credentials/iam/iam_credentials.cc

@@ -20,9 +20,6 @@
 
 #include "src/core/lib/security/credentials/iam/iam_credentials.h"
 
-#include "src/core/lib/gprpp/ref_counted_ptr.h"
-#include "src/core/lib/surface/api_trace.h"
-
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
 #include <grpc/support/string_util.h>
@@ -51,7 +48,10 @@ void grpc_google_iam_credentials::cancel_get_request_metadata(
 
 grpc_google_iam_credentials::grpc_google_iam_credentials(
     const char* token, const char* authority_selector)
-    : grpc_call_credentials(GRPC_CALL_CREDENTIALS_TYPE_IAM) {
+    : grpc_call_credentials(GRPC_CALL_CREDENTIALS_TYPE_IAM),
+      debug_string_(absl::StrFormat(
+          "GoogleIAMCredentials{Token:%s,AuthoritySelector:%s}",
+          token != nullptr ? "present" : "absent", authority_selector)) {
   grpc_mdelem md = grpc_mdelem_from_slices(
       grpc_slice_from_static_string(GRPC_IAM_AUTHORIZATION_TOKEN_METADATA_KEY),
       grpc_slice_from_copied_string(token));
@@ -62,9 +62,6 @@ grpc_google_iam_credentials::grpc_google_iam_credentials(
       grpc_slice_from_copied_string(authority_selector));
   grpc_credentials_mdelem_array_add(&md_array_, md);
   GRPC_MDELEM_UNREF(md);
-  debug_string_ = absl::StrFormat(
-      "GoogleIAMCredentials{Token:%s,AuthoritySelector:%s}",
-      token != nullptr ? "present" : "absent", authority_selector);
 }
 
 grpc_call_credentials* grpc_google_iam_credentials_create(

src/core/lib/security/credentials/iam/iam_credentials.h

@@ -39,11 +39,11 @@ class grpc_google_iam_credentials : public grpc_call_credentials {
 
   void cancel_get_request_metadata(grpc_credentials_mdelem_array* md_array,
                                    grpc_error* error) override;
-  std::string debug_string() const override { return debug_string_; }
+  std::string debug_string() override { return debug_string_; }
 
  private:
   grpc_credentials_mdelem_array md_array_;
-  std::string debug_string_;
+  const std::string debug_string_;
 };
 
 #endif /* GRPC_CORE_LIB_SECURITY_CREDENTIALS_IAM_IAM_CREDENTIALS_H */

src/core/lib/security/credentials/jwt/jwt_credentials.h

@@ -49,7 +49,7 @@ class grpc_service_account_jwt_access_credentials
   const gpr_timespec& jwt_lifetime() const { return jwt_lifetime_; }
   const grpc_auth_json_key& key() const { return key_; }
 
-  std::string debug_string() const override {
+  std::string debug_string() override {
     return absl::StrFormat("JWTAccessCredentials{ExpirationTime:%s}",
                            absl::FormatTime(absl::FromUnixMicros(
                                gpr_timespec_to_micros(jwt_lifetime_))));

src/core/lib/security/credentials/oauth2/oauth2_credentials.cc

@@ -357,7 +357,7 @@ grpc_oauth2_token_fetcher_credentials::grpc_oauth2_token_fetcher_credentials()
   grpc_httpcli_context_init(&httpcli_context_);
 }
 
-std::string grpc_oauth2_token_fetcher_credentials::debug_string() const {
+std::string grpc_oauth2_token_fetcher_credentials::debug_string() {
   return "OAuth2TokenFetcherCredentials";
 }
 
@@ -399,7 +399,7 @@ class grpc_compute_engine_token_fetcher_credentials
     grpc_resource_quota_unref_internal(resource_quota);
   }
 
-  std::string debug_string() const override {
+  std::string debug_string() override {
     return absl::StrFormat(
         "GoogleComputeEngineTokenFetcherCredentials{%s}",
         grpc_oauth2_token_fetcher_credentials::debug_string());
@@ -477,7 +477,7 @@ grpc_refresh_token_credentials_create_from_auth_refresh_token(
       refresh_token);
 }
 
-std::string grpc_google_refresh_token_credentials::debug_string() const {
+std::string grpc_google_refresh_token_credentials::debug_string() {
   return absl::StrFormat("GoogleRefreshToken{ClientID:%s,%s}",
                          refresh_token_.client_id,
                          grpc_oauth2_token_fetcher_credentials::debug_string());
@@ -555,7 +555,7 @@ class StsTokenFetcherCredentials
 
   ~StsTokenFetcherCredentials() override { grpc_uri_destroy(sts_url_); }
 
-  std::string debug_string() const override {
+  std::string debug_string() override {
     return absl::StrFormat(
         "StsTokenFetcherCredentials{Path:%s,Authority:%s,%s}", sts_url_->path,
         sts_url_->authority,
@@ -753,7 +753,7 @@ grpc_access_token_credentials::grpc_access_token_credentials(
   gpr_free(token_md_value);
 }
 
-std::string grpc_access_token_credentials::debug_string() const {
+std::string grpc_access_token_credentials::debug_string() {
   bool access_token_present = !GRPC_MDISNULL(access_token_md_);
   return absl::StrFormat("AccessTokenCredentials{Token:%s}",
                          access_token_present ? "present" : "absent");

src/core/lib/security/credentials/oauth2/oauth2_credentials.h

@@ -86,7 +86,7 @@ class grpc_oauth2_token_fetcher_credentials : public grpc_call_credentials {
 
   void on_http_response(grpc_credentials_metadata_request* r,
                         grpc_error* error);
-  std::string debug_string() const override;
+  std::string debug_string() override;
 
  protected:
   virtual void fetch_oauth2(grpc_credentials_metadata_request* req,
@@ -115,7 +115,7 @@ class grpc_google_refresh_token_credentials final
     return refresh_token_;
   }
 
-  std::string debug_string() const override;
+  std::string debug_string() override;
 
  protected:
   void fetch_oauth2(grpc_credentials_metadata_request* req,
@@ -143,7 +143,7 @@ class grpc_access_token_credentials final : public grpc_call_credentials {
   void cancel_get_request_metadata(grpc_credentials_mdelem_array* md_array,
                                    grpc_error* error) override;
 
-  std::string debug_string() const override;
+  std::string debug_string() override;
 
  private:
   grpc_mdelem access_token_md_;

src/core/lib/security/credentials/plugin/plugin_credentials.cc

@@ -42,7 +42,7 @@ grpc_plugin_credentials::~grpc_plugin_credentials() {
   }
 }
 
-std::string grpc_plugin_credentials::debug_string() const {
+std::string grpc_plugin_credentials::debug_string() {
   std::string debug_str;
   if (plugin_.debug_string != nullptr) {
     char* debug_c_str = plugin_.debug_string(plugin_.state);

src/core/lib/security/credentials/plugin/plugin_credentials.h

@@ -59,7 +59,7 @@ struct grpc_plugin_credentials final : public grpc_call_credentials {
   // cancelled before completion.
   void pending_request_complete(pending_request* r);
 
-  std::string debug_string() const override;
+  std::string debug_string() override;
 
  private:
   void pending_request_remove_locked(pending_request* pending_request);

test/cpp/end2end/end2end_test.cc

@@ -177,7 +177,7 @@ class TestMetadataCredentialsPlugin : public MetadataCredentialsPlugin {
     }
   }
 
-  grpc::string DebugString() const override {
+  grpc::string DebugString() override {
     return absl::StrFormat("TestMetadataCredentials{key:%s,value:%s}",
                            metadata_key_.c_str(), metadata_value_.c_str());
   }