Handle ConcurrentInvocations in User Event Service

User Event service currently handles the `ConcurrentRateLimit` and `TimedRateLimit` as simple counter but does not handle the `ConcurrentInvocations` which is more like a Gauge

We should add support for handling `ConcurrentInvocations` also. Each time any invocation starts an event is emitted which captures the current concurrent invocation count for namespace of the initiating user

```json
{
"body": {
"metricName": "ConcurrentInvocations",
"metricValue": 1
},
"eventType": "Metric",
"namespace": "guest",
"source": "controllerstandalone",
"subject": "guest",
"timestamp": 1571631891304,
"userId": "23bc46b1-71f6-4ed5-8c54-816aa4f8c502"
}
```
This count reflects the current concurrent invocation count for that controller. So to reflect the cluster wide concurrent activation count for given action we would need to maintain per cluster count and sum it up and then reflect that in metrics as a gauge