metrics: Add a gauge for the number of active blocking queries

[banks]

We currently have a metric for blocking queries but it's almost useless since it's a counter that increments whenever a blocking query begins:

consul/agent/consul/rpc.go

Line 495 in 973341a

metrics.IncrCounter([]string{"rpc", "query"}, 1)

What operators (and consul developer 😉 ) really want to know about is how many blocking queries are actually open/being handled right now.

This should be easy:

• add an uint64 to the server that is accessed atomically (watch struct alignment - probably needs to be at the top of the struct)
• atomically increment it right on the line linked above when a blocking query starts
• defer an atomic decrement for when the query returns
• after each operation emit a gauge metric with the current value

Note that we shouldn't replace or rename the current metric since that is a breaking change for users monitoring stacks, this should be a new gauge, I suggest we call it consul.rpc.queries_blocking or something (that bikeshed needs painting).

[mkcp]

Looking to pick this up later this week