Add metrics for otelhttp client

[vchitai]

Add some metrics for the otelhttp transport for http client net/http

Includes those metrics:

• http_client_duration

Tested sample

$ curl localhost:10080/metrics
# HELP http_client_duration
# TYPE http_client_duration histogram
http_client_duration_bucket{http_client_host="localhost:10080",http_client_method="GET",http_client_path="/hello1",http_client_status="200",http_host="localhost:10080",http_method="GET",http_path="/hello1",http_status="200",le="+Inf"} 2
http_client_duration_sum{http_client_host="localhost:10080",http_client_method="GET",http_client_path="/hello1",http_client_status="200",http_host="localhost:10080",http_method="GET",http_path="/hello1",http_status="200"} 0.645514
http_client_duration_count{http_client_host="localhost:10080",http_client_method="GET",http_client_path="/hello1",http_client_status="200",http_host="localhost:10080",http_method="GET",http_path="/hello1",http_status="200"} 2

Referencing: https://github.com/census-instrumentation/opencensus-go/blob/master/plugin/ochttp/stats.go

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Chi-Tai Vong (ccdcdd6, 755ead1, 5584474, d6a2098, 5f70e63)

[MrAlias]

The http metrics we implement need to conform to the OpenTelemetry metric semantic conventions: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/metrics/semantic_conventions/http-metrics.md

[vchitai]

The http metrics we implement need to conform to the OpenTelemetry metric semantic conventions: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/metrics/semantic_conventions/http-metrics.md

Thanks for the feedback, I will change it later.

[vchitai]

@MrAlias I have updated this PR according to your document, can you review it again?

[MrAlias]

Reading of the response body should not be included in the measurement of a requests duration. Right?

[vchitai]

The request will be counted as ended after we finish reading all the data from the response body, won't it?

[Aneurysm9]

There is probably utility in measurements of both time to first byte and time to the end of the body being read, though the latter is potentially less a measure of the client and more of whatever is processing the response data.

[vchitai]

This only wrap and returns reader interface rather than actual read and parsing actual data itself. The actual parsing time could be useful, but should not be included in this package, dont you think ?

[Aneurysm9]

Since this is the primary mechanism by which I expect otelhttp.Transport instances to be created and this leaves no configuration option for enabling/disabling metric collection, does it make more sense to integrate the metrics collection directly in the Transport.RoundTrip method? This would also be more in line with the approach taken by the otelhttp.Handler.

[vchitai]

I prefer to split up the implementation of metrics and tracing parts for ease of maintenance.

[MrAlias]

This should include the response status not http.StatusInternalServerError

[vchitai]

In case of HTTP connection error occurred, should we recorded nothing or 500 as the HTTP status code. What's your opinion?