Add metrics to button (part of #508)

[debnil]

This PR adds metrics functionality to Button.js. A subsequent PR will add the eventName to all invocations of Button.

[debnil]

@nikhilsaraf Two questions for you:

1. What are the "values" that we'd send, as part of the metric event? Of course, the data that already exists on the metricsTracker will be sent. But what else would we add on click.
2. I suspect that this sendMetricEvent() function will be used in many of the changes we are making. Where would be the best place to put it?

[nikhilsaraf]

can mark as non-draft so we can merge in the next round.

[nikhilsaraf]

seems like should be ready in the next round.

[nikhilsaraf]

closing because work included in #642