Provide a way to configure the ManagedChannelBuilder when building gRPC channels in InstantiatingGrpcChannelProvider

[steveniemitz]

The default InstantiatingGrpcChannelProvider has no way to configure the underlying ManagedChannelBuilder when building channels. Specifically, we have customizations we'd like to apply to the NettyChannelBuilder before it creates a ManagedChannel.

[michaelbausor]

@garrettjonesgoogle it seems reasonable to either support modifications like this, or else allow implementation of a custom TransportChannelProvider (right now it is marked @InternalExtensionOnly), WDYT?

[garrettjonesgoogle]

Yeah this seems like a good thing to add.

[igorbernstein2]

This would require a bit more work. InstantiatingGrpcChannelProvider decorates the channel with GrpcHeaderInterceptor and GrpcMetadataHandlerInterceptorboth of these are InternalApi. I think that before we allow users to create custom TransportChannelProviders we would need to pull those interceptors up to ClientContext.create().

[igorbernstein2]

Also as a parallel to the discussion in googleapis/java-bigtable-hbase#1906 (comment), maybe as opposed to exposing TransportChannelProvider for subclassing, we can expose ChannelPool and let users instantiate their own channels, wrap them in a ChannelPool and pass them via FixedTransportChannelProvider?