Dataflow Python SDK logging: step_id is always empty string

Using the dataflow runner, log messages always show up in stackdriver with the step_id as the empty string, so filtering log messages for a step doesn't work.
```

resource: {
labels: {
job_id: "" 
job_name: "" 
project_id: "" 
region: "" 
step_id: "" 
}
type: "dataflow_step" 
}
```

Another user seems to have posted in the old github repo and appears to be seeing the same problem based on their output:

[https://github.com/GoogleCloudPlatform/DataflowPythonSDK/issues/62](https://github.com/GoogleCloudPlatform/DataflowPythonSDK/issues/62)

From what I can tell is only affecting streaming pipelines

Imported from Jira [BEAM-7934](https://issues.apache.org/jira/browse/BEAM-7934). Original Jira may contain additional context.
Reported by: jimpremise.