FR: expose a getter for ChannelPoolSettings in InstantiatingGrpcChannelProvider

[ehsannas]

Is your feature request related to a problem? Please describe.
We'd like to have observability (e.g. through logs or traces) of the grpc channel pool settings. This can help identify potential misconfiguration of clients by customers.

Describe the solution you'd like
Expose a getter for ChannelPoolSettings in InstantiatingGrpcChannelProvider

Additional context
b/323070915

[ehsannas]

Thanks @lqiu96 !