Merge ~vtapia/maas:lp1927941 into maas:master

[Victor Tapia]

Reuse metric structures for Prometheus

After enabling Prometheus metrics, regiond was creating a new set of metrics + CollectorRegistry with every query. Those structures were never released, turning into a memory leak. This patch reuses the same metric structures which keep updating with every query.

[MAAS Lander (maas-lander)]

UNIT TESTS
-b lp1927941 lp:~vtapia/maas/+git/maas into -b master lp:~maas-committers/maas

STATUS: SUCCESS
COMMIT: 0036ade433495530beebebe2d0245f85c0987dd9

review: Approve

[Alberto Donato (ack)]

Thanks for the change, +1

One small nit inline

review: Approve

14b3bd0... by Victor Tapia on 2021-06-01

Apply naming convention on _METRICS

[MAAS Lander (maas-lander)]

UNIT TESTS
-b lp1927941 lp:~vtapia/maas/+git/maas into -b master lp:~maas-committers/maas

STATUS: FAILED
LOG: http://maas-ci.internal:8080/job/maas/job/branch-tester/10186/console
COMMIT: 14b3bd07af70f85328fdf587d14dea640a49a64f

review: Needs Fixing

[Victor Tapia (vtapia)]

Done, thanks for the review!

diff --git a/src/maasserver/prometheus/stats.py b/src/maasserver/prometheus/stats.py
index ff47b15..862ddb9 100644
--- a/src/maasserver/prometheus/stats.py
+++ b/src/maasserver/prometheus/stats.py
@@ -113,6 +113,8 @@ STATS_DEFINITIONS = [
     ),
 ]
 
+_METRICS = {}
+
 
 def prometheus_stats_handler(request):
     configs = Config.objects.get_configs(["prometheus_enabled", "uuid"])
@@ -120,14 +122,17 @@ def prometheus_stats_handler(request):
     if not have_prometheus:
         return HttpResponseNotFound()
 
-    metrics = create_metrics(
-        STATS_DEFINITIONS,
-        extra_labels={"maas_id": configs["uuid"]},
-        update_handlers=[update_prometheus_stats],
-        registry=prom_cli.CollectorRegistry(),
-    )
+    global _METRICS
+    if not _METRICS:
+        _METRICS = create_metrics(
+            STATS_DEFINITIONS,
+            extra_labels={"maas_id": configs["uuid"]},
+            update_handlers=[update_prometheus_stats],
+            registry=prom_cli.CollectorRegistry(),
+        )
+
     return HttpResponse(
-        content=metrics.generate_latest(), content_type="text/plain"
+        content=_METRICS.generate_latest(), content_type="text/plain"
     )