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]

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