Google Spanner monitor provides helpful information about databases and instance. Operation per seconds view contains errors(combined) measure that is not clear for me.
How to understand errors(combined)?
You can make a dashboard in Stackdriver (https://app.google.stackdriver.com) that will break down the errors slightly. We're working on a resources page for Cloud Spanner right now that will actually break them down by error code, but before that, you can go to Resources > Metrics Explorer and filter by response status:
You'll occasionally get error responses using the Cloud Spanner API; FAILED_PRECONDITION is somewhat common if you have a lot of transactions happening simultaneously that invalidate other transactions.