docs: add OTLP bridge tutorial for client_golang users

[happysnaker]

Summary

• add a dedicated tutorials/otlp-bridge guide
• show how to bridge a Prometheus Gatherer into the OpenTelemetry SDK
• link the new tutorial from the top-level README

Why

Issue #1495 asks for documentation explaining how existing client_golang instrumentation can be exported through an OTLP pipeline. The new tutorial gives users a concrete starting point without expanding the main README too much.

Related to #1495

[happysnaker]

Friendly follow-up: all checks are green on this one.

I kept the change intentionally small: a dedicated tutorial under tutorials/ plus a README link, so users get a concrete OTLP bridge example without adding too much detail to the top-level docs.

[happysnaker]

Resubmitted from a restored public fork as #2034. Closing this thread in favor of the new PR so the contribution stays reviewable from a proper fork relationship.