telemetry/module.py: update gather_perf_counters to emit values of labeled counters

telemetry/module.py: update gather_perf_counters to emit values of labeled counters

Until now, gather_perf_counters only included the values of unlabeled
counters. We update the API to include the values of labeled counters.
This change also updates the format of how these values are now emitted.

The perf counters in the telemetry report are now shows as below:
```
"perf_counters": {
        "mon.239f8ba73d60451130f972fcd40d9b409b1bfb66": {
            "AsyncMessenger::Worker": [
                {
                    "counters": {
                        "msgr_connection_idle_timeouts": {
                            "value": 0
                        },
                        "msgr_connection_ready_timeouts": {
                            "value": 0
                        }
                    },
                    "labels": {
                        "id": "0"
                    }
                }
            ],
```

Notice that each counter now has two new subfields: 'counters' and
'labels'.

Signed-off-by: Naveen Naidu <naveen.naidu@ibm.com>