Provide elapsed time when using 'timed' context manager

[tuukkamustonen]

Consider:

with statsd.timed('mymetric') as t:
    ...
t.ms  # get elapsed time

This is possible with statsd python package, but unfortunately not with datadog package (elapsed time is not stored in _TimedContextManagerDecorator. If I made a PR, would be willing to merge something like this:

    class _TimedContextManagerDecorator(object):

        def __init__(self, statsd, metric=None, tags=None, sample_rate=1, use_ms=None):
            ...
            self.elapsed = None

        def __enter__(self):
            ...
            return self

        def _send(self, start):
            self.elapsed = time() - start
            ...

And yeah, I could call time() - t.start myself, but it wouldn't match the sent value as time() could be different already...

[yannmh]

Nice idea @tuukkamustonen 💡 , I was not actually aware that such a method exists in the statsd Python package.
We'll be happy to review your PR if you decide to make one :)

[tuukkamustonen]

Fixed in #154.