Add JobCountByAllStates method to Client

[jcass8695]

Addresses #1017. In our organisation we would like to be able to have a regular metric published with the count of jobs in each state for monitoring purposes.

[brandur]

We don't normally expose driver functions like this without strong justification. In this case, counting can somewhat unintuitively become a bit of a problem because it can be very slow when dealing with enormous numbers of rows.

[jcass8695]

We don't normally expose driver functions like this without strong justification. In this case, counting can somewhat unintuitively become a bit of a problem because it can be very slow when dealing with enormous numbers of rows.

Hey @brandur, thanks for taking a look at this change so promptly. The motivation is that we would like to be able to publish a metric on the number of jobs in each state. If this isn't something that you and the other maintainers would like to expose through the client then it's not something critically important for us but it would be a nice to have. Lmk your thoughts.