* Allow VectorSelector.String() without matchers
Previously this method was panicking because it was trying to allocate a
slice with capacity -1. There's nothing saying that VectorSelector
should have matchers, and it's actually prepared to have zero matcher
strings, so it's worth checking instead of panicking.
Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>
This commit adds `@ <timestamp>` modifier as per this design doc: https://docs.google.com/document/d/1uSbD3T2beM-iX4-Hp7V074bzBRiRNlqUdcWP6JTDQSs/edit.
An example query:
```
rate(process_cpu_seconds_total[1m])
and
topk(7, rate(process_cpu_seconds_total[1h] @ 1234))
```
which ranks based on last 1h rate and w.r.t. unix timestamp 1234 but actually plots the 1m rate.
Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>
