Allow Gitaly to be configured to use histograms for GRPC method calls

Add support for GRPC Latency Histograms in Prometheus

By default this is option is off, but it can be enabled through the config.toml file, like so:

[prometheus]
grpc_latency_buckets = [0.001, 0.005, 0.025, 0.1, 0.5, 1.0, 10.0, 30.0, 60.0, 300.0, 1500.0]

The example values come from a discussion on Slack: https://gitlab.slack.com/archives/C3ER3TQBT/p1485528909001849