[EXPORTER] Allow to share gRPC clients between OTLP exporters.

[owent]

Fixes #3010

Changes

• Add APIs to allow to share gRPC clients between OTLP exporters.
• gRPC depends abseil-cpp, so we always enable abseil-cpp when have WITH_OTLP_GRPC
• Keep grpc::Channel in OtlpGrpcClient so we can share it between multiple exporters.

We can use OtlpGrpcClientFactory::Create() to create an OtlpGrpcClient. This client can then be passed to OtlpGrpcExporterFactory::Create(), OtlpGrpcLogRecordExporterFactory::Create(), and OtlpGrpcMetricExporterFactory::Create(). By doing this, all gRPC exporters created by these methods will share the same OtlpGrpcClient and utilize the same grpc::CompletionQueue and grpc::Channel.

When the exporters call Shutdown, the OtlpGrpcClient will only call ForceFlush and will not Shutdown unless all the exporters using this client object are also shutdown.We can use OtlpGrpcClientFactory::Create() to create OtlpGrpcClient now, and then pass it to OtlpGrpcExporterFactory::Create() , OtlpGrpcLogRecordExporterFactory::Create() and OtlpGrpcMetricExporterFactory::Create() , so all gRPC exporters created by these methods will share the same OtlpGrpcClient and use the same grpc::CompletionQueue and grpc::Channel.

When the exporters call Shutdown, the OtlpGrpcClient just call ForceFlush but do not Shutdown unless all the exporters use this client object are shutdown.

Usage example:

OtlpGrpcClientOptions grpc_opts;
grpc_opts.endpoint = "localhost:12345";
// ... other initializations

OtlpGrpcExporterOptions trace_opts = grpc_opts;
OtlpGrpcLogRecordExporterOptions logs_opts = grpc_opts;

nostd::shared_ptr<OtlpGrpcClient> shared_client = OtlpGrpcClientFactory::Create(OtlpGrpcLogRecordExporterOptions());
auto trace_exporter = OtlpGrpcExporterFactory::Create(trace_opts, shared_client);
auto logs_exporter = OtlpGrpcLogRecordExporterFactory::Create(logs_opts, shared_client);

// ...

For significant contributions please make sure you have completed the following items:

• [x] CHANGELOG.md updated for non-trivial changes
• [x] Unit tests have been added
• [ ] Changes in public API reviewed