HTTP metrics: reporting additional error reason when response was not received

[lmolkova]

Original discussion: open-telemetry/opentelemetry-specification#3243 (comment)

Problem:

• HTTP metrics have http.response.status_code attribute to measure error rate/count:

  • When an error happens before the response is received (SSL, DNS, timeout, connection refused, etc), no HTTP status code is reported.
  • Essentially, the lack of http.response.status_code means an error

• It's helpful to know more error details (timeout, DNS, SSL, etc) when no response is received. We can record it on spans in description, and in logs as an exceptions, but there is no way to record it on metrics.

• Adding a new attribute like this to metrics would be a breaking change as we'll split 'error' bucket into multiple new time series.

Proposal:

• introduce HTTP-specific extendable status code to represent an error reason.