fix(server): align routed request metric labels and outcomes

Count a local relay as successful only when its supervisor claims it, the
same event that answers a peer relay on the owner, and rename the outcomes
to local_error and remote_error so they say where an attempt failed. Label
the peer latency histogram by operation, like the routed request counter,
and rename the target label to relay_kind so it does not read as the
Prometheus scrape target.

Rename RelayCapacity.global to per_replica, drop the per-sandbox relay
capacity gauge, which no per-sandbox series can pair with, describe the
latency histogram as peer-only, and restore the note that unavailable
spikes are expected during rollouts.

Part of #3528

Signed-off-by: Emilien Macchi <emacchi@redhat.com>

Author: EmilienM