This PR adds batching support for GCP Observability logging. So instead
of the naive creating a new RPC to cloud logging for each logging event,
we now batch the log events to meet one of the following requirements -
* Batch size of 1000
* Batch memory consumption of 1MB
* A timeout period of 1sec after which we flush the accumulated batch
irrespective of the size.
There can also be cases where for some reason the RPCs fail or the batch
just accumulates to a very large size(100000 entries or 10MB in size).
In such cases, we just log the events with gpr_log instead of just
continuing to accumulate.
Additionally, `GcpObservabilityClose()` has been added to gracefully
shut off logging where we block till all the currently logged events are
flushed. (We might be able to gracefully shut off stats and tracing in
the future too.)
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