[monitoring/prometheus] No separator between MetricFactory.Prefix and metric names

[DazWilkin]

If not intentional (developer provides separator if wanted), consider inserting a separator here:

trillian/monitoring/prometheus/metrics.go

Line 39 in d4bf59f

Name: pmf.Prefix + name,

trillian/monitoring/prometheus/metrics.go

Line 61 in d4bf59f

Name: pmf.Prefix + name,

trillian/monitoring/prometheus/metrics.go

Line 97 in d4bf59f

Name: pmf.Prefix + name,

Perhaps simply:
Name: pmf.Prefix + "_" + name

Avoids e.g.:

# HELP someprefixmysql_queue_leaf_latency_entry Latency of insert-entry part of queue (single) leaf operation in seconds
# TYPE someprefixmysql_queue_leaf_latency_entry histogram

With:

# HELP someprefix_mysql_queue_leaf_latency_entry Latency of insert-entry part of queue (single) leaf operation in seconds
# TYPE someprefix_mysql_queue_leaf_latency_entry histogram

I've written a simple OpenCensus monitoring example. I'd used "[namespace]/[metric]" because I have been testing with Stackdriver monitoring but realize this is a poor assumption and should probably use "_" there too. Will be consistent with this Prometheus implementation.

[RJPercival]

I believe it was intentional, i.e. the developer is expected to end their prefix with a separator if desired. We should probably document that Prefix variable though, to state that explicitly.