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

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 services.

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.

Example URL: http://localhost:8080/api/overview/myNamespace/metric-filter?filter=jvm.threads.runnable.count,org.apache.http.conn.ClientConnectionManager.Client.pending-connections

![capture](https://cloud.githubusercontent.com/assets/7151335/4628988/ef6815b0-539e-11e4-81a9-0cf6c8235222.PNG)