Delay dynamic sampling when metrics extraction fails

[jjbayer]

We should not perform any dynamic sampling when metrics extraction fails for any reason (e.g. invalid config, outdated version), unless we are in a processing relay, which is the last possible hop that can perform dynamic sampling.

This can be done in two ways:

1. Return a Result from metrics extraction and rewrite the sampling decision to "keep" with sampling rate None if the result is an error.
2. Validate the metrics config before computing the sampling decision, and pass an infallible version of the config into the metrics extraction function, as described in

   relay/relay-server/src/processing/transactions/extraction.rs

   Lines 54 to 55 in 648cc5b

   	// TODO(follow-up): this function should always extract metrics. Dynamic sampling should validate
   	// the full metrics extraction config and skip sampling if it is incomplete.

TODO:

• Write an integration test that validates behavior.
• Fix the code.

[linear-code]

RELAY-268

[Dav1dde]

See also: #5403