Add a metric for # of exceptions per scan executor

[ddanielr]

Is your feature request related to a problem? Please describe.
Users are using the monitor logs in order to identify when exceptions are happening in user-defined iterator code.

Describe the solution you'd like
Servers should emit a metric for exceptions that are thrown from the iterator stack. The metric should be tagged with the executor name that was running it.

Additional context
This would probably also include hadoop related exceptions. I don't know if this could get separated out at a later date.
For now, this metric would provide the same functionality that the monitor logs view does today, enabling the deprecation of the logs view in future monitor versions.

[ArbaazKhan1]

I can take a look at this