Document what the observability stack costs in RPS #187

There is currently nothing in the docs about what turning an instrument on costs. It is a lot, and the ordering is not what people expect.

Measured on a trivial async def endpoint returning {"ok": True}, uvicorn single worker, ab -k -c 16 -n 20000. Apple M2, CPython 3.14.7, fastapi 0.141.1, sentry-sdk 2.67.1, opentelemetry-sdk 1.44.0, prometheus-fastapi-instrumentator 8.1.0.

Per instrument, measured alone, driving the ASGI app in-process so only library cost shows (baseline 15.8 μs/req):

Things a page like this should say, all of which surprised me:

• OpenTelemetry is twice Sentry, and is the dominant cost in the stack. Most people assume Sentry is the expensive one.
• LoggingInstrument is free until you actually log. The cost is per record (~8 μs with Sentry on), not per request.
• The Sentry knobs people reach for do nothing. attach_stacktrace=False, max_breadcrumbs=0 and dropping the default integrations all measured within noise. The entire cost is the ASGI integration, and over half of that is a Transaction built and discarded because sentry_traces_sample_rate is unset.
• sentry_traces_sample_rate=1.0 costs +274 μs/request on top of that (2486 RPS, −68% vs bare). Worth stating next to the setting.
• Each saving has a price, and the page should be explicit about it: no breadcrumbs on error events, no release health, no Sentry-side trace correlation, sampled-away OTel traces.

Suggested home: a "Performance" or "What observability costs" page under Introduction, linked from each instrument's docs, with a short "if you are RPS-constrained, start here" section.

Caveats to include: these ratios are for a handler that does nothing. A service doing real work per request (a DB round trip, a downstream call) will see a much smaller relative hit - the absolute per-request cost is what transfers, not the percentage.

The benchmark harness that produced these numbers is reproducible and I can attach it if useful. Sibling issues cover the two OpenTelemetry knobs and the Sentry ones.