feat(observability): auto-instrument httpx for outbound trace propagation

[ecthelion77]

✨ Feature / Enhancement PR

🔗 Epic / Issue

Closes #4155

Improves observability for outbound HTTP calls made by the gateway to upstream MCP servers and OAuth providers.

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🚀 Summary

This PR adds automatic OpenTelemetry instrumentation for httpx, the HTTP client used by the gateway for all outbound requests (MCP server communication, OAuth token exchanges, health checks). Without this, outbound HTTP calls are invisible in distributed traces — the trace stops at the gateway and does not propagate traceparent headers to upstream services.

Changes:

1. mcpgateway/observability.py: Adds opentelemetry-instrumentation-httpx initialization in setup_observability(), calling HTTPXClientInstrumentor().instrument() to auto-instrument all httpx.AsyncClient and httpx.Client instances.

2. gunicorn.config.py: Re-applies httpx instrumentation in post_fork(). Gunicorn's pre-fork model requires re-initialization of OTel instrumentors in each worker process. The implementation calls uninstrument() first to avoid double-instrumentation warnings, then instrument().

3. pyproject.toml: Adds opentelemetry-instrumentation-httpx as a dependency.

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🧪 Checks

• make lint passes
• make test passes
• CHANGELOG updated (if user-facing)

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📓 Notes

Why uninstrument() before instrument() in post_fork?

Gunicorn forks workers from the master process. If the master already instrumented httpx, the forked worker inherits the instrumented state. Calling instrument() again would raise warnings. The safe pattern is uninstrument() + instrument() to ensure clean re-initialization in each worker.

[ecthelion77]

Suggested labels: enhancement, COULD, python, observability