Document valid conditions for making assertions

[ywelsch]

The documentation (see https://esrally.readthedocs.io/en/stable/recipes.html?highlight=debug#checking-queries-and-responses) does not describe what fields are available and what conditions are supported in assertions on responses.

For example, I tried using assertions on a raw-request, which was just silently ignored by the system.

Further, it is unclear what fields are available and what comparison operators you can use.

[ywelsch]

Additionally, rally currently silently ignores assertions (just logging a DEBUG message), maybe this should be treated as a hard failure (see

rally/esrally/driver/runner.py

Line 399 in 067b27c

self.logger.debug("Skipping assertion check in [%s] as [%s] does not return a dict.", op_name, repr(self.delegate))

)