polymur-gateway: option to prepend the key name into the metrics namespace

[jamiealquiza]

Polymur-gateway keys are created with a keyname:key (example: customer-a:dk239gkgrk2). Add an option that will nest all metrics sent from a polymur-proxy instance using the key dk239gkgrk2 with customer-a. The metric webnode01.some-app.rate will become customer-a.webnode01.some-app.rate.