feat(argocd-image-updater): Optionally expose metrics (#1005)

* feat(argocd-image-updater): Optionally expose metrics

Signed-off-by: Jan Pieper <kontakt@jan-pieper.info>

* Apply suggestions from code review

Signed-off-by: Marco Kilchhofer <mkilchhofer@users.noreply.github.com>

* Add missing link references to README

Signed-off-by: Marco Kilchhofer <mkilchhofer@users.noreply.github.com>

Co-authored-by: Marco Kilchhofer <mkilchhofer@users.noreply.github.com>

charts/argocd-image-updater/templates/servicemonitor.yaml

@@ -0,0 +1,38 @@
+{{- if and .Values.metrics.enabled .Values.metrics.serviceMonitor.enabled }}
+apiVersion: monitoring.coreos.com/v1
+kind: ServiceMonitor
+metadata:
+  name: {{ include "argocd-image-updater.fullname" . }}-metrics
+  {{- with .Values.metrics.serviceMonitor.namespace }}
+  namespace: {{ . }}
+  {{- end }}
+  labels:
+    {{- include "argocd-image-updater.labels" . | nindent 4 }}
+    {{- with .Values.metrics.serviceMonitor.selector }}
+      {{- toYaml . | nindent 4 }}
+    {{- end }}
+    {{- with .Values.metrics.serviceMonitor.additionalLabels }}
+      {{- toYaml . | nindent 4 }}
+    {{- end }}
+spec:
+  endpoints:
+    - port: metrics
+      {{- with .Values.metrics.serviceMonitor.interval }}
+      interval: {{ . }}
+      {{- end }}
+      path: /metrics
+      {{- with .Values.metrics.serviceMonitor.relabelings }}
+      relabelings:
+        {{- toYaml . | nindent 8 }}
+      {{- end }}
+      {{- with .Values.metrics.serviceMonitor.metricRelabelings }}
+      metricRelabelings:
+        {{- toYaml . | nindent 8 }}
+      {{- end }}
+  namespaceSelector:
+    matchNames:
+      - {{ .Release.Namespace }}
+  selector:
+    matchLabels:
+      {{- include "argocd-image-updater.selectorLabels" . | nindent 6 }}
+{{- end }}