GCP Observability: Docs on Init (#32573)

<!--

If you know who should review your pull request, please assign it to
that
person, otherwise the pull request would get assigned randomly.

If your pull request is for a specific language, please add the
appropriate
lang label.

-->

include/grpcpp/ext/gcp_observability.h

@@ -23,6 +23,12 @@ namespace grpc {
 namespace experimental {
 
 // Initialize GCP Observability for gRPC.
+// This should be called before any other gRPC operations like creating a
+// channel, server, credentials etc.
+// The most common usage would call this at the top (or near the top) in main().
+// As an implementation detail, this properly initializes the OpenCensus stats
+// and tracing plugin, so applications do not need to perform any additional
+// gRPC C++ OpenCensus setup/registration to get GCP Observability for gRPC.
 absl::Status GcpObservabilityInit();
 
 // Gracefully shuts down GCP Observability.