Merge ~vtapia/maas:lp1927941-26 into maas:2.6

[Victor Tapia]

Reuse metric structures for Prometheus

Backport of 2ad24e0

[MAAS Lander (maas-lander)]

UNIT TESTS
-b lp1927941-26 lp:~vtapia/maas/+git/maas into -b 2.6 lp:~maas-committers/maas

STATUS: SUCCESS
COMMIT: 7b9c60178ec7f43f0b4bf6d88022d5092c59b5b2

review: Approve

[Adam Collard (adam-collard)]

review: Approve

[MAAS Lander (maas-lander)]

LANDING
-b lp1927941-26 lp:~vtapia/maas/+git/maas into -b 2.6 lp:~maas-committers/maas

STATUS: FAILED BUILD
LOG: http://maas-ci.internal:8080/job/maas/job/branch-tester/10327/consoleText

[MAAS Lander (maas-lander)]

LANDING
-b lp1927941-26 lp:~vtapia/maas/+git/maas into -b 2.6 lp:~maas-committers/maas

STATUS: FAILED BUILD
LOG: http://maas-ci.internal:8080/job/maas/job/branch-tester/10336/consoleText

diff --git a/src/maasserver/prometheus/stats.py b/src/maasserver/prometheus/stats.py
index 826eaa1..4ae81af 100644
--- a/src/maasserver/prometheus/stats.py
+++ b/src/maasserver/prometheus/stats.py
@@ -99,6 +99,8 @@ STATS_DEFINITIONS = [
         ['arches'])
 ]
 
+_METRICS = {}
+
 
 def prometheus_stats_handler(request):
     configs = Config.objects.get_configs(['prometheus_enabled', 'uuid'])
@@ -106,13 +108,15 @@ 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")
 
 
 def update_prometheus_stats(metrics: PrometheusMetrics):