Introduce method "Environment" to the Runners interface.

This way we can relate to which environment a runner belongs.

internal/runner/abstract_manager.go

@@ -64,10 +64,14 @@ func (n *AbstractManager) EnvironmentStatistics() map[dto.EnvironmentID]*dto.Sta
 			ID:                 int(e.ID()),
 			PrewarmingPoolSize: e.PrewarmingPoolSize(),
 			IdleRunners:        uint(e.IdleRunnerCount()),
-			UsedRunners:        0, // Adjusted later as the information is present in the specific runner manager.
+			UsedRunners:        0, // Increased below.
 		}
 	}
 
+	for _, r := range n.usedRunners.List() {
+		environments[r.Environment()].UsedRunners++
+	}
+
 	return environments
 }