Added telemetry around Catalog APIs

[diptanu]

Adding some metrics to indicate the rate of Catalog requests, and RPC failures. These metrics are useful for operators to understand the health of Consul from the perspective of the users.

cc/ @slackpad I didn't add support for labels. Is Consul doing tagged metrics like Nomad these days?

An example -

{
   Name: "consul.client.api.success.catalog_list_services",
   Count: 5,
   Sum: 5, 
   Min: 1,
   Max: 1,
   Mean: 1,
   Stddev: 0,
   Labels: {
        "node": "node-name"
   }
},

[slackpad]

Hey @diptanu this is looking good.

Is Consul doing tagged metrics like Nomad these days?

We do support labels and this is a good place to probably add some additional info.

[slackpad]

Should be error to be consistent.

[diptanu]

@slackpad All done! Added the node name as a label, so operators would be able to group the API errors/success/rates by nodes. Let me know if you need anything else here?

[slackpad]

LGTM