Enable BQ streaming inserts for particular docTypes

[jklukas]

Currently, we can run gcp-ingestion as BigQuery sink either in streaming mode or in file_loads mode. The entire job follows one path or the other.

I'd like to add a mixed mode that would operate mostly like the existing file_loads mode, but would send a subset of messages through the streaming path based on docType.

We'd add a --bqStreamingDocTypes runtime configuration option that we would set to:

bqStreamingDocTypes=activity-stream/impression-stats,activity-stream/spoc-fills

to allow the Pocket team to realize some low-latency use cases.

[jklukas]

The PR for this is merged. To put this into action we would want to update the BQ Sink template with:

--bqWriteMethod=mixed

And then for the structured-decoded_bq-sink, set runtime parameter:

bqStreamingDocTypes=activity-stream/impression-stats,activity-stream/spoc_fills,mobile/activation

[jklukas]

cc @whd

[jklukas]

We got hit by GoogleCloudPlatform/DataflowJavaSDK#632

Some more undocumented restricitons of runtime parameters.