opentelemetrytracer: Log (debug) the number of exported spans (#37828)

Commit Message: Log (debug) the number of exported spans 

Additional Description: 

Adds a debug log message containing the number of (about to be) exported
spans. This is helpful in troubleshooting for cases where the
configuration looks "correct" but no spans are seen in the tracing
back-end. The stats sink could be used, but that only exports via
OTLP/gRPC which is not always possible.
Risk Level: Low
Testing: Manual and Unit tests
Docs Changes:
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional [API
Considerations](https://github.com/envoyproxy/envoy/blob/main/api/review_checklist.md):]

---------

Signed-off-by: Joao Grassi <[email protected]>

source/extensions/tracers/opentelemetry/grpc_trace_exporter.cc

@@ -42,6 +42,7 @@ void OpenTelemetryGrpcTraceExporter::onFailure(Grpc::Status::GrpcStatus status,
 bool OpenTelemetryGrpcTraceExporter::log(const ExportTraceServiceRequest& request) {
   client_->send(service_method_, request, *this, Tracing::NullSpan::instance(),
                 Http::AsyncClient::RequestOptions());
+  OpenTelemetryTraceExporter::logExportedSpans(request);
   return true;
 }
 

source/extensions/tracers/opentelemetry/http_trace_exporter.cc

@@ -70,6 +70,8 @@ bool OpenTelemetryHttpTraceExporter::log(const ExportTraceServiceRequest& reques
   Http::AsyncClient::Request* in_flight_request =
       thread_local_cluster->httpAsyncClient().send(std::move(message), *this, options);
 
+  OpenTelemetryTraceExporter::logExportedSpans(request);
+
   if (in_flight_request == nullptr) {
     return false;
   }

source/extensions/tracers/opentelemetry/trace_exporter.h

@@ -29,6 +29,20 @@ class OpenTelemetryTraceExporter : public Logger::Loggable<Logger::Id::tracing>
    * @return false When sending the request failed.
    */
   virtual bool log(const ExportTraceServiceRequest& request) = 0;
+
+  /**
+   * @brief Logs as debug the number of exported spans.
+   *
+   * @param request The protobuf-encoded OTLP trace request.
+   */
+  void logExportedSpans(const ExportTraceServiceRequest& request) {
+    if (request.resource_spans(0).has_resource()) {
+      if (request.resource_spans(0).scope_spans(0).has_scope()) {
+        ENVOY_LOG(debug, "Number of exported spans: {}",
+                  request.resource_spans(0).scope_spans(0).spans_size());
+      }
+    }
+  }
 };
 
 using OpenTelemetryTraceExporterPtr = std::unique_ptr<OpenTelemetryTraceExporter>;