[POA-2309] Add new hostname reserved tag (#220)

Plan:
https://postmanlabs.atlassian.net/wiki/spaces/PO/pages/5603098777/Plan+New+Relic+Log+Integration#Another-option%3A-Use-hostname-to-get-the-pod-name

tags/reserved_tags.go

@@ -23,6 +23,9 @@ const (
 	// The version of the service(s) under observation, as defined by the
 	// user.
 	XAkitaServiceVersion Key = "x-akita-service-version"
+
+	// Hostname we get on running `hostname` command. Set in the agent.
+	XInsightsHostname Key = "x-insights-hostname"
 )
 
 // Generic CI tags