Instead of doing sanity checks in the Kubernetes provider, we just
accept any non-empty value from the annotation and rely on the server
part to filter out unknown rules.
This allows us to automatically stay in sync with the currently
supported Path matchers/modifiers.
A missing annotation would previously be handled in the default error
case, causing a noisy warning-level log message to be generated each
time.
We add another case statement to ignore the case where the annotation is
missing from the annotations map.
Also piggybacking a minor improvement to the log message.
