grpcproxy: use metadata instead of context withvalue in with client auth token

[krijohs]

Change to use metadata instead of context.WithValue to ensure each proxy watcher client has a new stream created with its token.
Previously context.WithValue resulted in streamKeyFromCtx returning an empty string in the clientv3 watcher, causing stream reuse.
When new clients connected to proxy after the token expired (token for the initial client which connected) the reused stream's context would still contain the expired token. This caused auth failures when isWatchPermitted on cluster checked the stream's context resulting in hanging proxy watcher clients.

Issue can be reproduced by setting a low --auth-token-ttl on cluster and connect 1 client first to proxy and then connect a second one after token expired.

Also adds increase of watchersCoalescing metric when watchers have been coalesced.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @krijohs. Thanks for your PR.

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[krijohs]

Added test case which reproduces the issue, with included change it passes but without fails