Cleanup ruby sampler metrics

* Use a simple counter for sampler duration instead of a histogram.
* Use a counter to collect GC time.
* Remove unused objects metric.
* Cleanup metric names to match Prometheus conventions.
* Prefix generic GC stats with `gc_stat`.
* Include worker label on memory and file descriptor metrics.

1 files changed, 14 insertions, 0 deletions

diff --git a/doc/administration/monitoring/prometheus/gitlab_metrics.md b/doc/administration/monitoring/prometheus/gitlab_metrics.md
index 411a0fae93f..cea6764df41 100644
--- a/doc/administration/monitoring/prometheus/gitlab_metrics.md
+++ b/doc/administration/monitoring/prometheus/gitlab_metrics.md
@@ -49,6 +49,20 @@ The following metrics are available:
 | filesystem_circuitbreaker         | Gauge     | 9.5   | Whether or not the circuit for a certain shard is broken or not |
 | circuitbreaker_storage_check_duration_seconds | Histogram | 10.3 | Time a single storage probe took |
 
+### Ruby metrics
+
+Some basic Ruby runtime metrics are available:
+
+| Metric                                 | Type      | Since | Description |
+|:-------------------------------------- |:--------- |:----- |:----------- |
+| ruby_gc_duration_seconds_total         | Counter   | 11.1  | Time spent by Ruby in GC |
+| ruby_gc_stat_...                       | Gauge     | 11.1  | Various metrics from [GC.stat] |
+| ruby_file_descriptors                  | Gauge     | 11.1  | File descriptors per process |
+| ruby_memory_bytes                      | Gauge     | 11.1  | Memory usage by process |
+| ruby_sampler_duration_seconds_total    | Counter   | 11.1  | Time spent collecting stats |
+
+[GC.stat]: https://ruby-doc.org/core-2.3.0/GC.html#method-c-stat
+
 ## Metrics shared directory
 
 GitLab's Prometheus client requires a directory to store metrics data shared between multi-process services.