Add support for client-side cert/key to gRPC exports.

TODO: Add tests.

I have not run or written any tests, other than verifying it worked once
in my production system.

api/CMakeLists.txt

@@ -113,6 +113,11 @@ if(WITH_OTLP_HTTP_SSL_PREVIEW)
   endif()
 endif()
 
+if (WITH_OTLP_GRPC_MTLS_PREVIEW)
+  target_compile_definitions(opentelemetry_api
+                             INTERFACE ENABLE_OTLP_GRPC_MTLS_PREVIEW)
+endif()
+
 if(WITH_METRICS_EXEMPLAR_PREVIEW)
   target_compile_definitions(opentelemetry_api
                              INTERFACE ENABLE_METRICS_EXEMPLAR_PREVIEW)

exporters/otlp/include/opentelemetry/exporters/otlp/otlp_grpc_exporter_options.h

@@ -3,6 +3,8 @@
 
 #pragma once
 
+#include "absl/types/optional.h"
+#include "absl/types/variant.h"
 #include "opentelemetry/exporters/otlp/otlp_environment.h"
 
 #include <memory>
@@ -28,6 +30,19 @@ struct OtlpGrpcExporterOptions
   // ssl_credentials_cacert_as_string in-memory string representation of .pem file to be used for
   // SSL encryption.
   std::string ssl_credentials_cacert_as_string = GetOtlpDefaultSslCertificateString();
+
+#ifdef ENABLE_OTLP_GRPC_MTLS_PREVIEW
+  // At most one of ssl_client_key_* should be non-empty. If use_ssl_credentials, they will
+  // be read to allow for mTLS.
+  std::string ssl_client_key_path   = GetOtlpDefaultTracesSslClientKeyPath();
+  std::string ssl_client_key_string = GetOtlpDefaultTracesSslClientKeyString();
+
+  // At most one of ssl_client_cert_* should be non-empty. If use_ssl_credentials, they will
+  // be read to allow for mTLS.
+  std::string ssl_client_cert_path   = GetOtlpDefaultTracesSslClientCertificatePath();
+  std::string ssl_client_cert_string = GetOtlpDefaultTracesSslClientCertificateString();
+#endif
+
   // Timeout for grpc deadline
   std::chrono::system_clock::duration timeout = GetOtlpDefaultTimeout();
   // Additional HTTP headers

exporters/otlp/src/otlp_grpc_client.cc

@@ -34,6 +34,16 @@ static std::string GetFileContents(const char *fpath)
   finstream.close();
   return contents;
 }
+
+// If the file path is non-empty, returns the contents of the file. Otherwise returns contents.
+static std::string GetFileContentsOrInMemoryContents(
+  const std::string& file_path, const std::string& contents) {
+  if (!file_path.empty()) {
+    return GetFileContents(file_path.c_str());
+  }
+  return contents;
+}
+
 }  // namespace
 
 std::shared_ptr<grpc::Channel> OtlpGrpcClient::MakeChannel(const OtlpGrpcExporterOptions &options)
@@ -58,17 +68,20 @@ std::shared_ptr<grpc::Channel> OtlpGrpcClient::MakeChannel(const OtlpGrpcExporte
   grpc::ChannelArguments grpc_arguments;
   grpc_arguments.SetUserAgentPrefix(options.user_agent);
 
-  if (options.use_ssl_credentials)
-  {
-    grpc::SslCredentialsOptions ssl_opts;
-    if (options.ssl_credentials_cacert_path.empty())
-    {
-      ssl_opts.pem_root_certs = options.ssl_credentials_cacert_as_string;
-    }
-    else
-    {
-      ssl_opts.pem_root_certs = GetFileContents((options.ssl_credentials_cacert_path).c_str());
-    }
+  if (options.use_ssl_credentials) {
+    grpc::SslCredentialsOptions ssl_opts = {
+      .pem_root_certs = GetFileContentsOrInMemoryContents(
+          options.ssl_credentials_cacert_path,
+          options.ssl_credentials_cacert_as_string),
+#if ENABLE_OTLP_GRPC_MTLS_PREVIEW
+      .pem_private_key = GetFileContentsOrInMemoryContents(
+          options.ssl_client_key_path,
+          options.ssl_client_key_string),
+      .pem_cert_chain = GetFileContentsOrInMemoryContents(
+          options.ssl_client_cert_path,
+          options.ssl_client_cert_string)
+#endif
+    };
     channel =
         grpc::CreateCustomChannel(grpc_target, grpc::SslCredentials(ssl_opts), grpc_arguments);
   }