SURFSARA: add a new service to Reva to expose desired metrics

[labkode]

This task will produce a new HTTP service that will expose the metrics described in:
#8

Create a new service using the hello service as an example:
https://github.com/cs3org/reva/tree/master/internal/http/services/helloworld.

For example:
https://github.com/cs3org/reva/tree/master/internal/http/services/cs3meshmetrics.

[rontrompert]

HTTP service will expose the following metrics:

1. Number of users per site
2. Number of groups per site
3. Amount of used storage per site
4. Number of local shares per site
5. Number of shares between federated sync-and -share services
6. Local application usage per site
7. Cross-site application usage