ACC-444 new method: display given metric values for all known services

- added new method
api/overview/acrolinxcloud/metric-filter?filter=jvm.prop.key1,jvm.prop2.key2

In the current dashboard, you can see all metrics values for one
specific service easily. What's missing is a way to see one metric value
for all known service.
Use cases: 
- Show me the free heap space for all services
- Show me a custom metric like a fixed service-version gauge for all
services

This commit adds the backend part of the functionality. It is already
useable by calling the rest resource directly. In a later stage, there
could be an extra page in the dashboard.
Merge pull request #9 from fexbraun/get-metrics-value-for-all-services

ACC-444 new method: display given metric values for all known services