This PR adds cluster members to the metrics API. The number of members per
segment are reported as well as the total number of members.
Tested by running a multi-node cluster locally and ensuring the numbers were
correct. Also added unit test coverage to add the new expected gauges to
existing test cases.
Previously, we would emit service usage metrics both with and without a
namespace label attached. This is problematic in the case when you want
to aggregate metrics together, i.e. "sum(consul.state.services)". This
would cause services to be counted twice in that aggregate, once via the
metric emitted with a namespace label, and once in the metric emited
without any namespace label.
Using the newly provided state store methods, we periodically emit usage
metrics from the servers.
We decided to emit these metrics from all servers, not just the leader,
because that means we do not have to care about leader election flapping
causing metrics turbulence, and it seems reasonable for each server to
emit its own view of the state, even if they should always converge
rapidly.
