GCP Observability: Fix constant labels support for stats when tracing is disabled (#32451)

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include/grpcpp/opencensus.h

@@ -174,6 +174,9 @@ class CensusContext {
                          const ::opencensus::tags::TagMap& tags)
       : span_(::opencensus::trace::Span::StartSpan(name)), tags_(tags) {}
 
+  explicit CensusContext(const ::opencensus::tags::TagMap& tags)
+      : span_(::opencensus::trace::Span::BlankSpan()), tags_(tags) {}
+
   CensusContext(absl::string_view name, const ::opencensus::trace::Span* parent,
                 const ::opencensus::tags::TagMap& tags)
       : span_(::opencensus::trace::Span::StartSpan(name, parent)),

src/cpp/ext/filters/census/client_filter.cc

@@ -294,13 +294,11 @@ OpenCensusCallTracer::OpenCensusCallTracer(
       method_(GetMethod(path_)),
       arena_(arena),
       tracing_enabled_(tracing_enabled) {
-  if (tracing_enabled_) {
-    auto* parent_context = reinterpret_cast<CensusContext*>(
-        call_context_[GRPC_CONTEXT_TRACING].value);
-    GenerateClientContext(
-        absl::StrCat("Sent.", method_), &context_,
-        (parent_context == nullptr) ? nullptr : parent_context);
-  }
+  auto* parent_context = reinterpret_cast<CensusContext*>(
+      call_context_[GRPC_CONTEXT_TRACING].value);
+  GenerateClientContext(tracing_enabled_ ? absl::StrCat("Sent.", method_) : "",
+                        &context_,
+                        (parent_context == nullptr) ? nullptr : parent_context);
 }
 
 OpenCensusCallTracer::~OpenCensusCallTracer() {
@@ -319,13 +317,6 @@ OpenCensusCallTracer::~OpenCensusCallTracer() {
   }
 }
 
-void OpenCensusCallTracer::GenerateContext() {
-  auto* parent_context = reinterpret_cast<CensusContext*>(
-      call_context_[GRPC_CONTEXT_TRACING].value);
-  GenerateClientContext(absl::StrCat("Sent.", method_), &context_,
-                        (parent_context == nullptr) ? nullptr : parent_context);
-}
-
 OpenCensusCallTracer::OpenCensusCallAttemptTracer*
 OpenCensusCallTracer::StartNewAttempt(bool is_transparent_retry) {
   // We allocate the first attempt on the arena and all subsequent attempts on
@@ -363,7 +354,7 @@ void OpenCensusCallTracer::RecordAnnotation(absl::string_view annotation) {
 }
 
 CensusContext OpenCensusCallTracer::CreateCensusContextForCallAttempt() {
-  if (!tracing_enabled_) return CensusContext();
+  if (!tracing_enabled_) return CensusContext(context_.tags());
   GPR_DEBUG_ASSERT(context_.Context().IsValid());
   auto context = CensusContext(absl::StrCat("Attempt.", method_),
                                &(context_.Span()), context_.tags());

src/cpp/ext/filters/census/context.cc

@@ -42,6 +42,11 @@ void GenerateServerContext(absl::string_view tracing, absl::string_view method,
   // Destruct the current CensusContext to free the Span memory before
   // overwriting it below.
   context->~CensusContext();
+  if (method.empty()) {
+    new (context) CensusContext(grpc::internal::OpenCensusRegistry::Get()
+                                    .PopulateTagMapWithConstantLabels({}));
+    return;
+  }
   SpanContext parent_ctx =
       opencensus::trace::propagation::FromGrpcTraceBinHeader(tracing);
   if (parent_ctx.IsValid()) {
@@ -62,6 +67,11 @@ void GenerateClientContext(absl::string_view method, CensusContext* ctxt,
   // Destruct the current CensusContext to free the Span memory before
   // overwriting it below.
   ctxt->~CensusContext();
+  if (method.empty()) {
+    new (ctxt) CensusContext(grpc::internal::OpenCensusRegistry::Get()
+                                 .PopulateTagMapWithConstantLabels({}));
+    return;
+  }
   if (parent_ctxt != nullptr) {
     SpanContext span_ctxt = parent_ctxt->Context();
     Span span = parent_ctxt->Span();

src/cpp/ext/filters/census/server_filter.cc

@@ -141,9 +141,11 @@ OpenCensusServerCallData::OpenCensusServerCallData(
   FilterInitialMetadata(client_initial_metadata, &sml);
   path_ = std::move(sml.path);
   method_ = GetMethod(path_);
-  if (OpenCensusTracingEnabled()) {
-    GenerateServerContext(sml.tracing_slice.as_string_view(),
-                          absl::StrCat("Recv.", method_), &context_);
+  auto tracing_enabled = OpenCensusTracingEnabled();
+  GenerateServerContext(
+      tracing_enabled ? sml.tracing_slice.as_string_view() : "",
+      absl::StrCat("Recv.", method_), &context_);
+  if (tracing_enabled) {
     auto* call_context = grpc_core::GetContext<grpc_call_context_element>();
     call_context[GRPC_CONTEXT_TRACING].value = &context_;
   }

test/cpp/ext/filters/census/constant_labels_wait_test.cc

@@ -49,6 +49,7 @@ class ConstantLabelsWaitTest : public StatsPluginEnd2EndTest {
  protected:
   static void SetUpTestSuite() {
     grpc::internal::OpenCensusRegistry::Get().RegisterWaitOnReady();
+    grpc::internal::EnableOpenCensusTracing(false);
     StatsPluginEnd2EndTest::SetUpTestSuite();
   }
 };