Add proxy metrics documentation (#536)

* Add proxy metrics documentation

Signed-off-by: Alex Leong <alex@buoyant.io>

doc/proxy-metrics.md

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++++
+title = "Proxy Metrics"
+docpage = true
+[menu.docs]
+  parent = "proxy-metrics"
++++
+
+The Conduit proxy exposes metrics that describe the traffic flowing through the
+proxy.  The following metrics are available at `/metrics` on the proxy's metrics
+port (default: `:4191`) in the [Prometheus format][prom-format]:
+
+### `request_total`
+
+A counter of the number of requests the proxy has received.  This is incremented
+when the request stream begins.
+
+### `request_duration_ms`
+
+A histogram of the duration of a request.  This is measured from when the
+request headers are received to when the request stream has completed.
+
+### `response_total`
+
+A counter of the number of responses the proxy has received.  This is
+incremeneted when the response stream ends.
+
+### `response_duration_ms`
+
+A histogram of the duration of a response.  This is measured from when the
+response headers are received to when the response stream has completed.
+
+### `response_latency_ms`
+
+A histogram of the total latency of a response.  This is measured from when the
+request headers are received to when the response stream has completed.
+
+## Labels
+
+Each of these metrics has the following labels:
+
+* `direction`: `inbound` if the request originated from outside of the pod,
+               `outbound` if the request originated from inside of the pod.
+* `deployment`: The deployment that the pod belongs to (if applicable).
+* `job`: The job that the pod belongs to (if applicable).
+* `replica_set`: The replica set that the pod belongs to (if applicable).
+* `daemon_set`: The daemon set that the pod belongs to (if applicable).
+* `replication_controller`: The replication controller that the pod belongs to 
+                            (if applicable).
+* `authority`: The value of the `:authority` (HTTP/2) or `Host` (HTTP/1.1)
+               header of the request.
+* `dst_deployment`: The deployment to which this request is being sent.  Only
+                    applicable if `direction=outbound`.
+* `dst_job`: The job to which this request is being sent.  Only applicable if
+             `direction=outbound`.
+* `dst_replica_set`: The replica set to which this request is being sent.  Only
+                     applicable if `direction=outbound`.
+* `dst_daemon_set`: The daemon set to which this request is being sent.  Only
+                    applicable if `direction=outbound`.
+* `dst_replication_controller`: The replication controller to which this request
+                                is being sent.  Only applicable if
+                                `direction=outbound`.
+* `dst_namespace`: The namespace to which this request is being sent.  Only
+                   applicable if `direction=outbound`.
+* `status_code`: The HTTP status code of the response.  Only applicable to
+                 response metrics.
+* `grpc_status_code`: The value of the `grpc-status` trailer.  Only applicable
+                      to response metrics for gRPC responses.
+
+Note that the `instance` and `namespace` labels will typically be added by the
+Prometheus collector.
+
+[prom-format]: https://prometheus.io/docs/instrumenting/exposition_formats/#format-version-0.0.4