Add counters for monitoring

[willbryant]

Verm should count the following events, per execution:

• Number of GET requests served
• Number of GET requests served successfully (ie. 200 and 304)
• Number of GET requests to paths not found (ie. 404)
• Number of POST file requests (new file stores) served
• Number of POST file requests (new file stores) served resulting in a new file store
• Number of PUT file requests (replication stores) served
• Number of PUT file requests (replication stores) served resulting in a new file store

These statistics should be exposed via GET request to a special path.

[willbryant]

Closed by 00ff65b. Went with GET requests failed rather than GET requests successful because successful usually means 2xx and does not include 3xx.