hyperledger / fabric-x-committer / 30253020917

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[sidecar, query] Add metric for active client connections (#706)

#### Type of change

- New feature
- Improvement (improvement to code)

#### Description

- Add a gRPC `stats.Handler` (`serve.ConnStatsHandler`) that tracks the
number of
open server-side connections in a Prometheus gauge - incrementing on
`ConnBegin`
  and decrementing on `ConnEnd`. It is attached to every gRPC server.
- A service opts in by registering a gauge from its own provider via
`serve.RegisterConnStatHandler` in `RegisterService`. Sidecar and query
do so,
exposing `sidecar_grpc_active_connections` and
`queryservice_grpc_active_connections`
  on their existing metrics endpoints.
- The gauge is held in an `atomic.Pointer`, so registering it is safe
even if it
  races the server handling connections.

#### Related issues

- resolves #705

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Signed-off-by: Dean Amar <Dean.Amar@ibm.com>

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