Some timestamp functions can be safely wrapped as a step invariant.
Then once we do that we never need to check or unwrap at evaluation time.
For instance; `timestamp(metric @ 1)` is step invariant, whereas
`timestamp(abs(metric @ 1))` is not.
Currently all `timestamp` `*parser.Call` are excluded from being
considered step invariant since it is listed in the
`AtModifierUnsafeFunctions` map.
This PR adds an extra check for timestamp functions which consider
the arguments, and if the argument is a simple VectorSelector then
the entire `*parser.Call` can be safely wrapped as a step invariant.
Signed-off-by: Andrew Hall <andrew.hall@grafana.com>
Co-authored-by: Bryan Boreham <bjboreham@gmail.com>
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