docs(meter): Update meter.Meter contract

[cep21]

Resolves #4836

[cep21]

Documenting that callers can rely on the current behavior of meter.Meter, which is different than the rest of the Go standard library. For example, http.NewRequest returns a nil first object if the second return value, error, is non nil.

[MrAlias]

This is too restrictive. There is nothing in the OTel metric specification that states an implementation cannot return nil and an error from one of these methods. Restricting all implementations of this interface to always return a non-nil value is overreaching.

[MrAlias]

Closing based on the resolution to #4836 being open-telemetry/opentelemetry-go-contrib#4875.