Implement timestamp() for native histograms

[harry671003]

Notes

• Implementing timestamp() for native histograms
• Timestamp function should return a float sample with t=T, v=T/1000 for both native histograms and floats

From the docs: https://prometheus.io/docs/prometheus/latest/querying/functions/#timestamp

timestamp()
timestamp(v instant-vector) returns the timestamp of each of the samples of the given vector as the number of seconds since January 1, 1970 UTC. It acts on float and histogram samples in the same way.

[fpetkovski]

Why do we need these changes? Can't apply the same pattern to histograms as we do for floats?

[harry671003]

When we have both samples and histograms in the input StepVector, we need to return a StepVector with only Samples populated.

Eg:

load 30s
  http_request_duration_seconds{pod="nginx-1"} {{schema:0 count:3 sum:14.00 buckets:[1 2]}}+{{schema:0 count:4 buckets:[1 2 1]}}x20
  http_request_duration_seconds{pod="nginx-2"} 1x10 {{schema:0 count:2 sum:14.00 buckets:[2]}}+{{schema:0 count:6 buckets:[2 2 2]}}x10`

I'm not sure if we should do this in place.

for _, vector := range in {
		for i := range vector.Samples {
			vector.Samples[i] = float64(vector.T / 1000)
		}
		for i := range vector.Histograms {
			vector.Samples = append(vector.Samples, float64(vector.T)/1000)
			vector.SampleIDs = append(vector.SampleIDs, vector.HistogramIDs[i])
		}
		vector.Histograms = nil
		vector.HistogramIDs = nil
	}
}

[harry671003]

Could you take another look @fpetkovski?

[fpetkovski]

@harry671003 would you mind rebasing this so we can merge it?