Plot the MongoDB flow control

MongoDB added a flow control mechanism that will be helpful to graph

https://docs.mongodb.com/manual/tutorial/troubleshoot-replica-sets/#flow-control

These are all the metrics that mongodb_exporter v2 exports from the [diagnosticData.]serverStatus.flowControl section for v4.2 mongod nodes.

N.b. no info in these values from secondary nodes. "Flow control" is a throttle that runs on the primary, it's only the primary that will have increments in these counters.

We've never graphed these before because we didn't have the exporter for it. So there's some discovery to do here, but I think we only need graphs for two of these:

• mongodb_ss_flowControl_isLaggedTimeMicros
  When the slowest secondary starts lagging significantly enough flowControl will begin, and this will start to grow. Will be a fraction of wall clock time and reaching 100% of wall clock time will indicate that is completely engaged and blocking all client ops.

• mongodb_ss_flowControl_timeAcquiringMicros
  Cumulative time client ops are forced to wait by flowcontrol. E.g. if a hundred ops are blocked it would grow by 100s (or rather, 100 million microsecs) per second of wall clock time.

Nb. according to https://docs.mongodb.com/manual/reference/command/serverStatus/#serverstatus.flowControl.locksPerKiloOp locksPerOp is deprecated as of v4.4.