Configure Prometheus to allow a custom scrape query param

* Set `prometheus.io/param` on a Kubernetes Service to scrape
the service endpoints and pass a custom query parameter
* For example, scrape Consul with `?format=prometheus`

```yaml
kind: Service
metadata:
  annotations:
    prometheus.io/scrape: 'true'
    prometheus.io/port: '8500'
    prometheus.io/path: /v1/agent/metrics
    prometheus.io/param: format=prometheus
```

CHANGES.md

@@ -11,6 +11,10 @@ Notable changes between versions.
 
 * Switch Kubernetes Container Runtime from `docker` to `containerd` ([#1087](https://github.com/poseidon/typhoon/pull/1087))
 
+### Addons
+
+* Configure Prometheus to allow a custom scrape query parameter ([#1095](https://github.com/poseidon/typhoon/pull/1095))
+
 ## v1.23.0
 
 * Kubernetes [v1.23.0](https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.23.md#v1230)

addons/prometheus/config.yaml

@@ -175,6 +175,7 @@ data:
     # * `prometheus.io/path`: If the metrics path is not `/metrics` override this.
     # * `prometheus.io/port`: If the metrics are exposed on a different port to the
     # service then set this appropriately.
+    # * `prometheus.io/param`: Custom metrics query parameter, like "format=prometheus".
     - job_name: 'kubernetes-service-endpoints'
       kubernetes_sd_configs:
       - role: endpoints
@@ -197,6 +198,11 @@ data:
         target_label: __address__
         regex: ([^:]+)(?::\d+)?;(\d+)
         replacement: $1:$2
+      - source_labels: [__meta_kubernetes_pod_annotation_prometheus_io_param]
+        action: replace
+        target_label: __param_$1
+        regex: ([^=]+)=(.*)
+        replacement: $2
       - action: labelmap
         regex: __meta_kubernetes_service_label_(.+)
       - source_labels: [__meta_kubernetes_namespace]