Feature Request: include information in VTGate LogStats that can indicate if a scatter is happening

[maxenglander]

Feature Description

A common request among users of the Vitess clusters my organization operates is to know when queries are resulting in scatters.

vtgate/logstats.LogStats has a ShardQueries field, which is great, but not all multi-shard queries are scatters.

Possible ways this could be handled:

• Add a numeric ScatterQueries field to LogStats incremented when a query scatters.
• Add a more generic Opcode field to LogStats set to Scatter when the engine opcode is such.

Use Case(s)

For some users, an unintentional scatter may be the result of improper schema or query design, and they would like to know about them so they can adjust their schema or queries.