Report CallDroppedException in exception metrics

`CallDroppedException` can be thrown with `CallRunner.drop()` by queue implementations that decide to drop calls to groom the RPC call backlog. The LifoCoDel queue does this I believe and with Pluggable queue it's possible for 3rd party queue implementations to be using `drop()` for similar reasons (my use case). It would be nice for the server to be tracking these exceptions in metrics since otherwise you might have to do some extra lifting on the client side.

I have a patch I will submit soon.