Solving http.request.method cardinality (and similar) with metrics hints API

[lmolkova]

Original issue: open-telemetry/opentelemetry-specification#3470

TL;DR: http.request.method can have high cardinality, especially when a malicious or buggy client sends requests with invalid methods. Servers don't have control over this behavior.

The problem is generic and could be solved with hint API that

• allows applications to specify known/prioritized values to record
• SDK follows hint API and records known values, and records unknown ones with 'unknown'/'other' (TBD) value

In the context of http.request.method:

• general-purpose instrumentation libraries SHOULD configure a hint with standard method names
• SDK users MAY override it and allow additional methods

This approach is outlined in #35

Pros:

• generic solution not specific to one attribute

Cons:

• cannot be applied to other signals if they need to have low cardinality (e.g. span names)
• can't query across signals for attributes with capped values.

[lmolkova]

This can now be closed as we went ahead with approach in #17