GcpObservability: Add config support for constant labels (#31710)

src/cpp/ext/gcp/observability_config.h

@@ -97,6 +97,7 @@ struct GcpObservabilityConfig {
   absl::optional<CloudMonitoring> cloud_monitoring;
   absl::optional<CloudTrace> cloud_trace;
   std::string project_id;
+  std::map<std::string, std::string> labels;
 
   static const grpc_core::JsonLoaderInterface* JsonLoader(
       const grpc_core::JsonArgs&) {
@@ -108,6 +109,7 @@ struct GcpObservabilityConfig {
                            &GcpObservabilityConfig::cloud_monitoring)
             .OptionalField("cloud_trace", &GcpObservabilityConfig::cloud_trace)
             .OptionalField("project_id", &GcpObservabilityConfig::project_id)
+            .OptionalField("labels", &GcpObservabilityConfig::labels)
             .Finish();
     return loader;
   }

test/cpp/ext/gcp/observability_config_test.cc

@@ -55,7 +55,12 @@ TEST(GcpObservabilityConfigJsonParsingTest, Basic) {
       "cloud_trace": {
         "sampling_rate": 0.05
       },
-      "project_id": "project"
+      "project_id": "project",
+      "labels": {
+        "SOURCE_VERSION": "v1",
+        "SERVICE_NAME": "payment-service",
+        "DATA_CENTER": "us-west1-a"
+      }
     })json";
   auto json = grpc_core::Json::Parse(json_str);
   ASSERT_TRUE(json.ok()) << json.status();
@@ -88,6 +93,11 @@ TEST(GcpObservabilityConfigJsonParsingTest, Basic) {
   EXPECT_TRUE(config.cloud_trace.has_value());
   EXPECT_FLOAT_EQ(config.cloud_trace->sampling_rate, 0.05);
   EXPECT_EQ(config.project_id, "project");
+  EXPECT_THAT(config.labels,
+              ::testing::UnorderedElementsAre(
+                  ::testing::Pair("SOURCE_VERSION", "v1"),
+                  ::testing::Pair("SERVICE_NAME", "payment-service"),
+                  ::testing::Pair("DATA_CENTER", "us-west1-a")));
 }
 
 TEST(GcpObservabilityConfigJsonParsingTest, Defaults) {
@@ -103,6 +113,7 @@ TEST(GcpObservabilityConfigJsonParsingTest, Defaults) {
   EXPECT_FALSE(config.cloud_monitoring.has_value());
   EXPECT_FALSE(config.cloud_trace.has_value());
   EXPECT_TRUE(config.project_id.empty());
+  EXPECT_TRUE(config.labels.empty());
 }
 
 TEST(GcpObservabilityConfigJsonParsingTest, LoggingConfigMethodIllegalSlashes) {