Convert http://drupal.org/metrics to be driven by sampler metrics

Now that Sampler API is deployed and collecting data on Drupal.org, we'll want to convert http://drupal.org/metrics to use it for consistency. sampler already provides three of the five metrics on that page by default, so some of the work is already done. Remaining is:

• Write sampler metrics for 'Commits created by month' and 'Committers by month'. we'll need to figure out where these metrics will live, it would be great if they were general enough to live in one of the versioncontrol modules.
• Backfill data for the two metrics above. The other three have already been backfilled as part of #1497332: backfill metrics collected by Sampler API
• Rework the page to use views_sparkline

Deployment

• Upgrade sampler module
• drush sampler-update-storage-plugin drupalorg drupalorg_user_metric drupal_database_table_per_metric
• drush sampler-update-method-plugin drupalorg drupalorg_user_metric periodic
• drush sampler-sample drupalorg drupalorg_user_metric --startstamp=1010275200