Add performance statistics to Onionoo's servlet

n8fr8 plans to release an Orbot version that queries relay information via Onionoo. There are quite a few Orbots deployed, so this might put some load on Onionoo. We should add some simple performance statistics to Onionoo to detect load problems early.

To start with, let's add statistics for the following request, which is the first request that n8fr8 plans to add to Orbot (it's also the standard request that Atlas and Globe make to show relay details):

https://onionoo.torproject.org/details?lookup=$fingerprint

Some simple metrics to start with might be:

• Number of requests per 1, 5, 15 minutes
• Time to parse request, time to build response, time to send response

What I'm not sure about yet is where to store and display these statistics. We could start with heartbeat log messages containing these numbers.