The relative priority for requests. Note that priority is not applicable
for [BeginTransaction][google.spanner.v1.Spanner.BeginTransaction].
The priority acts as a hint to the Cloud Spanner scheduler and does not
guarantee priority or order of execution. For example:
Some parts of a write operation always execute at PRIORITY_HIGH,
regardless of the specified priority. This may cause you to see an
increase in high priority workload even when executing a low priority
request. This can also potentially cause a priority inversion where a
lower priority request will be fulfilled ahead of a higher priority
request.
If a transaction contains multiple operations with different priorities,
Cloud Spanner does not guarantee to process the higher priority
operations first. There may be other constraints to satisfy, such as
order of operations.
