Make contention_metric available for monitoring

[milesgranger]

Makes contention_metric a continuously updated value,
so long as the monitoring thread is running.

Example from one of the updated tests:

        # usually ~0.9 on linux ~0.6 on windows
        assert knocker.contention_metric > 0.6

        # Now wait for it to 'cool' back down
        # by looping over some work which releases the GIL
        prev_cm = knocker.contention_metric
        for i in range(10):
            a_little_gil()
            assert knocker.contention_metric < prev_cm
            prev_cm = knocker.contention_metric

        # ~0.15 oN mY MaChInE.
        assert knocker.contention_metric < 0.2

Ref: dask/distributed#7290 (comment)