Benchmarks: make benchmarks always report results for one operation

[roji]

Some of our benchmark perform multiple operations per method invocation, but do not report that do BDN; this means that the absolute numbers reported don't reflect a single operation. Use the [OperationsPerInvoke] attribute to flow this information out.

To avoid messing up our dashboard, we'd need to update historical numbers accordingly in the database.

Attempted in #20821 but never merged, bring that PR back.