Manager Instrumentation

[aluzzardi]

The manager is going to need instrumentation.

Instrumenting would provide interesting ops metrics, such as amount of tasks scheduled, scheduling time, etc etc.

This will be extremely useful to ops (e.g. graph scheduling time, page me if raft transactions take more than 5 seconds at 99th over the past hour, etc etc).

The hard part is deciding what to instrument, as to how, using the prometheus client library is probably a good start.

[aluzzardi]

Tentatively assigning this to you @stevvooe

[aluzzardi]

@crosbymichael - what's the status of go-metrics? Should we start using it now or should we wait?