[JAMES-3405] Expose metrics over HTTP

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 3.6.0
• Component/s: guice, Metrics, webadmin
• Assignee: Unassigned
• Reporter: Benoit Tellier
• Created: 06/Oct/20 07:20
• Updated: 09/Oct/20 10:43
• Resolved: 09/Oct/20 10:43

Description

= Why ?

I want to export James metrics to prometheus.

This modern metric stack relies on pulling (prometheus collects the metrics) instead of pushing.

This need had been expressed by Ioan Eugen Stan here: https://github.com/ieugen/james-self-hosting-sandbox/issues/20

= How ?

With the https://github.com/prometheus/client_java dependency expose that as part of a `webadmin-metrics` Unauthentictated endpoint.

Adapt content at https://github.com/prometheus/client_java/blob/master/simpleclient_servlet/src/main/java/io/prometheus/client/exporter/MetricsServlet.java

Be aware that there is some conversion format to be done here:

CollectorRegistry collectorRegistry = CollectorRegistry.defaultRegistry;
new DropwizardExports(environment.metrics()).register(collectorRegistry);
new MetricsServlet(collectorRegistry)

Source https://stackoverflow.com/questions/53408121/which-metrics-are-exported-by-dropwizardmetrics-prometheus-client

= Definition of done

As an admin,
I have an HTTP endpoint
Exposing metrics with the prometheus format

Scope: all guice products

This endpoint should require no authentication in order to be easily fetchable by prometheus.

However as an admin, I need to explicitly enable it in the webadmin configuration, it is disabled by default.