opentelemetrytracer: avoid exporting when there are no spans (#36313)

Commit Message: opentelemetrytracer: avoid exporting when there are no
spans
Additional Description: Today, the OpenTelemetry tracer exports a OTLP
request on each interval, even when there are no spans to be sent. The
OTLP is empty, only containing the resource (and its attributes).
Risk Level: Low
Testing: Manual
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #35997]
[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 <5938087+joaopgrassi@users.noreply.github.com>

changelogs/current.yaml

@@ -8,6 +8,9 @@ minor_behavior_changes:
 
 bug_fixes:
 # *Changes expected to improve the state of the world and are unlikely to have negative effects*
+- area: tracing
+  change: |
+    Fixed a bug where the OpenTelemetry tracer exports the OTLP request even when no spans are present.
 
 removed_config_or_runtime:
 # *Normally occurs at the end of the* :ref:`deprecation period <deprecated>`

source/extensions/tracers/opentelemetry/tracer.cc

@@ -163,6 +163,10 @@ void Tracer::enableTimer() {
 }
 
 void Tracer::flushSpans() {
+  if (span_buffer_.empty()) {
+    return;
+  }
+
   ExportTraceServiceRequest request;
   // A request consists of ResourceSpans.
   ::opentelemetry::proto::trace::v1::ResourceSpans* resource_span = request.add_resource_spans();