Function timeouts can cause slow perf / no scaling in HTTP

(For HTTP)

When a function timeout occurs, concurrently executing functions in a single instance will also be stopped (resulting in 502's). If the function timeout value is configured to a low value (many function executions are expected to timeout), performance and availability of all calls will be expected. We have seen function apps not scale correctly due to timeouts that are too aggressively configured. Function timeout values should only be used to stop executions that are outliers in execution time.

We should either provide better documentation on what happens on a timeout, add detectors for alerting of a possible issue around timeouts, or provide some sort of safe guard against this.

**Workaround:**
It is recommended that you configure a timeout value that is _not likely to occur_ frequently. The timeout value should be used to stop outlier execution times.

No file or test is identified. Start by reviewing the HTTP function timeout behavior described here and the documented workaround, then determine whether the accepted scope is documentation, alerting detectors, or a safeguard. Done means the selected response clearly addresses concurrent executions, 502 responses, and scaling impact when timeouts occur.

Written by the indexing model from the issue text.