Add input_error metric to redpanda input

[Pavelsky89]

Hi all,

The redpanda output has output_error metric exposed when it fails to send data.

For some reason, I don't see equivalent for the redpanda input. You may reproduce issue by configuring input to read from non existing topic, or from topic to which user has no access to. In that case in logs I can see "re-updating due to inner errors: UNKNOWN_TOPIC_OR_PARTITION", but there is no any metric based on which I could determine that something is not working

On the other hand, if I use non-existing topic in the redpanda output, then I get the output_error metric counter increased as expected

Is this missing feature or a bug, or is it by purpose?