Refactor/952 add metric for spanner pool get

[ethowitz]

Description

Adds a timer that tracks the time it takes to get a new Spanner pool. It seemed like this was the right approach based on other areas in the code we're logging elapsed time. I think we'll need to add the new timer to this chart or make a new chart entirely on the same dashboard.

Testing

I was able to test this by swapping out the QueuingMetricSync here with a SpyMetricSync that prints to a file like so:

let file = File::create("foo.txt")?;
let spy = SpyMetricSink::from(Arc::new(Mutex::new(file)));
StatsdClient::builder(opts.statsd_label.as_ref(), spy)

The metrics that would have been sent to sentry were logged to a file named "foo.txt", and I was able to confirm that the metric I added worked as expected. I included a commit (b658de0) on this branch with these changes so the reviewer can try it out themselves.

I didn't see unit tests for the metrics logging, so I didn't add one for this change, but it's totally possible I just couldn't find them!

Issue(s)

Closes #952