kas: metrics for monitoring

kas should expose Prometheus metrics. Look at labkit (https://godoc.org/gitlab.com/gitlab-org/labkit/metrics). Also https://godoc.org/gitlab.com/gitlab-org/labkit/monitoring

What metrics ?

kas:

• Groundwork - !94 (merged), !96 (merged).
• gRPC server metrics: per-rpc, per-status code counters - !97 (merged)
• gRPC client metrics: per-rpc, per-status code counters - !97 (merged)
• counters for number of long running connections (per RPC) - !99 (merged)
• counters for number of Gitaly polls (error, success, new commit). Covered by "gRPC client metrics" above.
• ...