opentelemetry-semantic-conventions: handle explicit buckets for metrics helpers

[xrmx]

Helpers that creates metrics should have knowledge of the preferred bucket boundaries otherwise they do more harm than good when used. Two that comes to mind are create_http_server_request_duration and create_http_client_request_duration. Wondering if the explicit buckets are found in what weaver generates (or even in the spec) and if they are referenced from the metrics.