Streaming watermark should be easier to read

Currently, the only way to get at the streaming watermarks is through scraping counter names. However, the watermarks are useful for determining if a streaming job is 'done,' ie watermarks at infinity. We should consider either exposing the watermarks through a GetWatermarks api or another alternative might be a WATERMARKS_AT_INFINITY job state.

Imported from Jira [BEAM-1170](https://issues.apache.org/jira/browse/BEAM-1170). Original Jira may contain additional context.
Reported by: joshualitt@google.com.

Start with the issue description and the linked BEAM-1170 Jira ticket, which may contain additional context. Compare the proposed GetWatermarks API and WATERMARKS_AT_INFINITY job state, then identify a concrete scope and completion criteria before implementation.

Written by the indexing model from the issue text.