We have a number of stats already expressed using the statsd interface. During
the switchover period to direct Prometheus collection, we'd like to make those
stats available both ways. This change automatically exports any stats exported
using the statsd interface via Prometheus as well.
This is a little tricky because Prometheus expects all stats to by registered
exactly once. Prometheus does offer a mechanism to gracefully recover from
registering a stat more than once by handling a certain error, but it is not
safe for concurrent access. So I added a concurrency-safe wrapper that creates
Prometheus stats on demand and memoizes them.
In the process, made a few small required side changes:
- Clean "/" from method names in the gRPC interceptors. They are allowed in
statsd but not in Prometheus.
- Replace "127.0.0.1" with "boulder" as the name of our testing CT log.
Prometheus stats can't start with a number.
- Remove ":" from the CT-log stat names emitted by Publisher. Prometheus stats
can't include it.
- Remove a stray "RA" in front of some rate limit stats, since it was
duplicative (we were emitting "RA.RA..." before).
Note that this means two stat groups in particular are duplicated:
- Gostats* is duplicated with the default process-level stats exported by the
Prometheus library.
- gRPCClient* are duplicated by the stats generated by the go-grpc-prometheus
package.
When writing dashboards and alerts in the Prometheus world, we should be careful
to avoid these two categories, as they will disappear eventually. As a general
rule, if a stat is available with an all-lowercase name, choose that one, as it
is probably the Prometheus-native version.
In the long run we will want to create most stats using the native Prometheus
stat interface, since it allows us to use add labels to metrics, which is very
useful. For instance, currently our DNS stats distinguish types of queries by
appending the type to the stat name. This would be more natural as a label in
Prometheus.
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