Add envoy metrics discovery bundle

[atoulme]

Description:
Adds a new discovery bundle targeting envoy proxy metrics.

[atoulme]

I tested this locally. It worked well enough on my machine.

[jinja2]

Is the k8s rule intentionally matching just pod name instead of the broader rule used by docker_observer? Istio sidecars for e.g. have pod name like istio-proxy, the image name might have envoy in it. But maybe we should only match standalone envoy to startwith

[atoulme]

Right, you had a note about that. I think we can update the rule to add istio-proxy but then I really need a test showing this works for an istio mesh.

[atoulme]

I am tempted to make a separate integration test with a kind cluster, we deploy everything and we make sure it works. I might get into that next and tweak the rule based on that.

[jinja2]

I think we should merge this one as-is. And have a different item to officially support istio. It is not just the pod name; default prom port used by istio is also not same as the one here. Don't know if it is supported in deiscover mode, but it would be better getting the port/path from the prometheus.io annotations for istio (like we have in our helm chart).

[jinja2]

Discover mode config test TestDockerObserver is failing

        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -49,3 +49,25 @@
        	            	    (string) (len=26) "receiver_creator/discovery": (map[string]interface {}) (len=2) {
        	            	-    (string) (len=9) "receivers": (map[string]interface {}) (len=2) {
        	            	+    (string) (len=9) "receivers": (map[string]interface {}) (len=3) {
        	            	+     (string) (len=10) "prometheus": (map[string]interface {}) (len=3) {
        	            	+      (string) (len=6) "config": (map[string]interface {}) (len=1) {
        	            	+       (string) (len=6) "config": (map[string]interface {}) (len=1) {
        	            	+        (string) (len=14) "scrape_configs": ([]interface {}) (len=1) {
        	            	+         (map[string]interface {}) (len=5) {
        	            	+          (string) (len=8) "job_name": (string) (len=5) "envoy",
        	            	+          (string) (len=22) "metric_relabel_configs": ([]interface {}) (len=1) {
        	            	+           <max depth reached>
        	            	+          },
        	            	+          (string) (len=12) "metrics_path": (string) (len=17) "/stats/prometheus",
        	            	+          (string) (len=15) "scrape_interval": (string) (len=3) "10s",
        	            	+          (string) (len=14) "static_configs": ([]interface {}) (len=1) {
        	            	+           <max depth reached>
        	            	+          }
        	            	+         }
        	            	+        }
        	            	+       }
        	            	+      },
        	            	+      (string) (len=19) "resource_attributes": (map[string]interface {}) {
        	            	+      },
        	            	+      (string) (len=4) "rule": (string) (len=121) "type == \"container\" and any([name, image, command], {# matches \"(?i)envoy\"}) and not (command matches \"splunk.discovery\")"
        	            	+     },
        	            	      (string) (len=17) "prometheus_simple": (map[string]interface {}) (len=3) {
        	Test:       	TestDockerObserver