Add simple HTTP metrics for both requests and outputs

[CheyiLin]

Hi,

Just added simple metrics for relay requests (HTTP server) and relay outputs (HTTP client).
It uses opencensus libs to provide prometheus compatible metrics on :8088/metrics.

Feel free to let me know if there is something I can improve.
Thank you.

• Example for http requests: relay_http_*

...
relay_http_request_bytes_bucket{http_method="POST",http_path="/api/v1/prom/write",http_status="204",le="1.073741824e+09"} 2332
relay_http_request_bytes_bucket{http_method="POST",http_path="/api/v1/prom/write",http_status="204",le="4.294967296e+09"} 2332
relay_http_request_bytes_bucket{http_method="POST",http_path="/api/v1/prom/write",http_status="204",le="+Inf"} 2332
relay_http_request_bytes_sum{http_method="POST",http_path="/api/v1/prom/write",http_status="204"} 6.2660830000000065e+06
relay_http_request_bytes_count{http_method="POST",http_path="/api/v1/prom/write",http_status="204"} 2332
# HELP relay_http_response_count_by_status_code Server response count by status code
# TYPE relay_http_response_count_by_status_code counter
relay_http_response_count_by_status_code{http_method="POST",http_path="/api/v1/prom/write",http_status="204"} 2332

• Example for outputs: relay_http_output_*

...
relay_http_output_roundtrip_latency_bucket{http_client_host="influxdb0:8086",http_client_method="POST",http_client_path="/api/v1/prom/write",http_client_status="204",le="50000"} 2332
relay_http_output_roundtrip_latency_bucket{http_client_host="influxdb0:8086",http_client_method="POST",http_client_path="/api/v1/prom/write",http_client_status="204",le="100000"} 2332
relay_http_output_roundtrip_latency_bucket{http_client_host="influxdb0:8086",http_client_method="POST",http_client_path="/api/v1/prom/write",http_client_status="204",le="+Inf"} 2332
relay_http_output_roundtrip_latency_sum{http_client_host="influxdb0:8086",http_client_method="POST",http_client_path="/api/v1/prom/write",http_client_status="204"} 10531.3364
relay_http_output_roundtrip_latency_count{http_client_host="influxdb0:8086",http_client_method="POST",http_client_path="/api/v1/prom/write",http_client_status="204"} 2332
# HELP relay_http_output_completed_count Count of completed requests, by HTTP method and response status
# TYPE relay_http_output_completed_count counter
relay_http_output_completed_count{http_client_host="influxdb0:8086",http_client_method="POST",http_client_path="/api/v1/prom/write",http_client_status="204"} 2332
relay_http_output
_completed_count{http_client_host="influxdb1:8086",http_client_method="POST",http_client_path="/api/v1/prom/write",http_client_status="204"} 2332

• Full Metrics

[CheyiLin]

I've improved the metric names for better readability:

• metrics with prefix relay_http_* are related to config section [[http]]
• metrics with prefix relay_http_output_* are related to config sections [[http.output]]