feat: add experimental http.{server/client}.request.duration metrics to instrumentation

[xuan-cao-swi]

Description

Add optional OpenTelemetry HTTP metrics support for http.server.request.duration and http.client.request.duration while preserving backward compatibility for Ruby users until the metrics SDK matures.

Prometheus dashboard (metrics come from sinatra app that made net http request call)

• Base
  • Added initialize_metrics hook that instrumentations should override to create instruments (e.g., counters, histograms).
    • initialize_metrics is invoked from install.
    • If no metrics SDK is installed/initialized or metrics are disabled, initialize_metrics does nothing.
  • Metrics are controlled by enable_metrics?, which considers configuration and an environment variable (metrics_env_var_value).
    • Default: disabled. Enable explicitly via config (e.g., {:metrics => true}) or env var.
• net_http (client)
  • Added a Metrics module that is prepended as a patch to record client request duration.
• rack (server)
  • Added EventHandlerWithMetrics and TracerMiddlewareWithMetrics to capture and record server-side request durations.

Python and JS instrumentations require metrics; Ruby’s metrics SDK is still developing. This design records metrics safely without forcing changes to existing behavior or requiring a full SDK.