Add changelog, fix compiling problems without async

Author: owent

CHANGELOG.md

@@ -21,6 +21,9 @@ Increment the:
 * [REMOVAL] Remove build option `WITH_DEPRECATED_SDK_FACTORY`
   [#2717](https://github.com/open-telemetry/opentelemetry-cpp/pull/2717)
 
+* [EXPORTER] Allow to share gRPC clients between OTLP exporters.
+  [#3041](https://github.com/open-telemetry/opentelemetry-cpp/pull/3041)
+
 Breaking changes:
 
 * [REMOVAL] Remove build option `WITH_DEPRECATED_SDK_FACTORY`

exporters/otlp/src/otlp_grpc_exporter.cc

@@ -94,8 +94,12 @@ std::unique_ptr<sdk::trace::Recordable> OtlpGrpcExporter::MakeRecordable() noexc
 sdk::common::ExportResult OtlpGrpcExporter::Export(
     const nostd::span<std::unique_ptr<sdk::trace::Recordable>> &spans) noexcept
 {
+#ifdef ENABLE_ASYNC_EXPORT
   nostd::shared_ptr<OtlpGrpcClient> client = client_;
   if (isShutdown() || !client)
+#else
+  if (isShutdown())
+#endif
   {
     OTEL_INTERNAL_LOG_ERROR("[OTLP gRPC] Exporting " << spans.size()
                                                      << " span(s) failed, exporter is shutdown");

exporters/otlp/src/otlp_grpc_log_record_exporter.cc

@@ -108,8 +108,12 @@ OtlpGrpcLogRecordExporter::MakeRecordable() noexcept
 opentelemetry::sdk::common::ExportResult OtlpGrpcLogRecordExporter::Export(
     const nostd::span<std::unique_ptr<opentelemetry::sdk::logs::Recordable>> &logs) noexcept
 {
+#ifdef ENABLE_ASYNC_EXPORT
   nostd::shared_ptr<OtlpGrpcClient> client = client_;
   if (isShutdown() || !client)
+#else
+  if (isShutdown())
+#endif
   {
     OTEL_INTERNAL_LOG_ERROR("[OTLP gRPC log] Exporting " << logs.size()
                                                          << " log(s) failed, exporter is shutdown");

exporters/otlp/src/otlp_grpc_metric_exporter.cc

@@ -98,8 +98,12 @@ sdk::metrics::AggregationTemporality OtlpGrpcMetricExporter::GetAggregationTempo
 opentelemetry::sdk::common::ExportResult OtlpGrpcMetricExporter::Export(
     const opentelemetry::sdk::metrics::ResourceMetrics &data) noexcept
 {
+#ifdef ENABLE_ASYNC_EXPORT
   nostd::shared_ptr<OtlpGrpcClient> client = client_;
   if (isShutdown() || !client)
+#else
+  if (isShutdown())
+#endif
   {
     OTEL_INTERNAL_LOG_ERROR("[OTLP METRICS gRPC] Exporting "
                             << data.scope_metric_data_.size()