Separate update pings and update requests graphs for Tor Browser alpha and release users

On https://metrics.torproject.org/webstats-tb.html we have some graphs showing the number of Update pings and Update requests from Tor Browser users.

Those numbers currently include both alpha and release users. However it would be nice to be able to know how many alpha users we have. For that we could have separate graphs for alpha and release users.

According to https://metrics.torproject.org/stats.html#webstats-tb the update_pings and update_requests are counted with:

• update_pings: GET requests to all sites with resource strings '%/torbrowser/update\__/%' and response code 200. To separate alpha and release users we could use the resource strings '%/torbrowser/update\__/alpha/%' and '%/torbrowser/update\__/release/%'.
• update_requests: GET requests to all sites with resource strings '%/torbrowser/%.mar' and response code 302. Separating update_requests is more difficult as the URL does not include the word alpha or release. However it includes the version number (as the name of the directory under /torbrowser/), which includes an a in case of an alpha version, and no letter (only numbers and dots) in case of a stable release.