chore: Replace channel provider for fixed channel provider.

[nimf]

Currently, when creating a stub, the channel provider creates a new channel pool every time, thus we end up with four channel pools to the same target instead of one.

In this PR we create a single channel pool using the existing channel provider and then wrap it into FixedTransportChannelProvider so that any stub using the fixed channel provider uses the created pool instead of creating a new one.

@surbhigarg92 do I also need to change anything in the metrics according to this change?

[olavloite]

I think that we need a bit more management and logic around this:

1. We should only use a FixedTransportChannelProvider if the user has not specified a channel provider themselves. That is; We should only do this if (options.getChannelProvider() == null). The reason is that for the default channel provider, we know that it produces a channel pool, which can safely be wrapped in a FixedChannelProvider. We don't know what any custom channel provider that has been set might produce, and whether it is a good idea to just get one channel from that provider and use that for all requests.
2. We also need to keep a reference to the channel that is returned by defaultChannelProvider.build().getTransportChannel(), and then close that channel when this GapicSpannerRpc instance is shut down. A FixedTransportChannelProvider is not closed when a stub is closed. This will leave the channels open after this instance has been closed.