Add membership queue metrics

[Half-Shot]

Fixes #259

This PR creates 3 new metrics entries for prometheus:

• membershipqueue_pending(type) which will show which types of membership are waiting in the queue
• membershipqueue_processed(type, outcome) will show the number of completed (success & failed) membership actions
• membershipqueue_reason(type, errcode, http_status) will be a count of the failures for the actions
• membershipqueue_lastage is the age of the last processed request, in MS

Output looks good:

# HELP bridge_membershipqueue_pending Count of membership actions in the queue by type
# TYPE bridge_membershipqueue_pending gauge
bridge_membershipqueue_pending{type="leave"} 0
bridge_membershipqueue_pending{type="kick"} 0
bridge_membershipqueue_pending{type="join"} 0

# HELP bridge_membershipqueue_processed Count of membership actions processed by type and outcome
# TYPE bridge_membershipqueue_processed counter
bridge_membershipqueue_processed{type="leave",outcome="success"} 6
bridge_membershipqueue_processed{type="kick",outcome="success"} 4
bridge_membershipqueue_processed{type="join",outcome="success"} 1

# HELP bridge_membershipqueue_reason Count of failures to process membership, by matrix errcode and http status
# TYPE bridge_membershipqueue_reason counter

# HELP bridge_membershipqueue_lastage Gauge to measure the age of the last processed event
# TYPE bridge_membershipqueue_lastage gauge
bridge_membershipqueue_lastage 0

[jaller94]

Looks good to me.