add RequestMetricsBuilder.with_metric_attrs_from_req

[dtheodor]

Adding a builder method with_metric_attrs_from_req that accepts a function to generate metric attributes from the request. Function has the same signature with and by default is util::metrics_attributes_from_request. Also exposing metrics_attributes_from_request since whoever wants to add additional attributes may want to reuse it and extend its result.

Fixes #171