[Feature Request]: Make Neo4jIO.WriteUnwind output write statistics.

[fbiville]

What would you like to happen?

This is one of the two blockers preventing Neo4jIO from being used in the Neo4j Flex template for GCP Dataflow.

Currently, WriteUnwind is implemented as a true sink.

As such, it cannot be depended upon (via Wait.on).

Dependencies are major part of the Neo4j Template.

WriteUnwind could output write statistics it gets from the Neo4j driver's org.neo4j.driver.summary.ResultSummary class (org.neo4j.driver.summary.SummaryCounters in particular).

Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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