Metrics reports an invalid Stack in Linux Consumption

#### Investigative information

In Linux mesh failover image, we set the FUNCTIONS_WORKER_RUNTIME_VERSION = 3.6
https://github.com/Azure/azure-functions-docker-private/blame/master/host/2.0/stretch/amd64/mesh.Dockerfile#L117

This causes the Python dashboard to show some weird Stack value in
```
WawsAn_omgsitefunctionsentity
| where pdate between (datetime(2020-01-18) .. datetime(2020-01-31))
| where Stack contains "3.6"
| sample 10000
| distinct Stack

-3.6
dotnet-3.6
node-3.6
powershell-3.6
python-3.6
```

#### Possible fix
https://github.com/Azure/azure-functions-host/commit/34a650012408448e5398eeab3edf8c5658c5ef7d#diff-312918ec1cd07ca8bb15fe41eb8836b6R277

Only acquire runtimeVersion when FUNCTIONS_WORKER_RUNTIME is set to "python".