[Task]: Add warnings and docuementation bout using SpannerIO.EeadAll in a streaming pipeline.

[nielm]

What needs to happen?

Using SpannerIO.ReadAll in a streaming pipeline has negative effects and is probably not what the customer wants to do:

• Streaming pipelines run effectively forever (unless manually stopped)
• SpannerIO.ReadAll creates a session and ReadOnlyTransaction on pipeline startup, and uses it for the rest of the pipeline duration.
• This will mean that all data read from spanner will be 'stale' and from the timestamp when the pipeline was first started.
• and if no reads occur for more than an hour, the session and transaction will be auto-closed from the spanner server side, causing the pipeline to fail.

Adding warnings to and documentation to SpannerIO.ReadAll about using it in a streaming pipeline and the negative side-effect

Issue Priority

Priority: 2 (default / most normal work should be filed as P2)

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