mozilla/browserid — pull request #2578 by seanmonstar (2012-10-10 22:52:20+00:00)
Title: adds ability to adjust KPI rate base on User Agent

In config, we can do something like this:

```js
"kpi_backend_sample_rate_per_agent": {
"Firefox/19.0": 0.4
}
```

To check the UA string for Firefox 19, and give it a 40 percent rate.

/cc @jedp, since you filed the initial bug

fixes #1931