pkg/parcacol: Group all profiles of a write request into one buffer

[brancz]

$ benchstat old.txt new.txt
name                              old time/op    new time/op    delta
ProfileColumnStoreWriteSeries-10    2.74ms ± 4%    0.00ms ± 6%  -99.99%  (p=0.000 n=10+10)

name                              old alloc/op   new alloc/op   delta
ProfileColumnStoreWriteSeries-10    4.05MB ± 0%    0.00MB ± 0%  -99.99%  (p=0.000 n=10+10)

name                              old allocs/op  new allocs/op  delta
ProfileColumnStoreWriteSeries-10     32.3k ± 0%      0.0k ± 0%  -99.98%  (p=0.000 n=10+10)

[metalmatze]

Suggested change

	func sortedKeys(m map[string]struct{}) []string {
	keys := make([]string, 0, len(m))
	for k := range m {
	keys = append(keys, k)
	}
	sort.Strings(keys)
	return keys
	}
	func sortedKeys(in map[string]struct{}) []string {
	if len(in) == 0 {
	return []string{}
	}
	out := maps.Keys(in)
	sort.Strings(out)
	return out
	}

Sadly the sort.Sort isn't generic yet, but when it's going to be, there will only be one of this ever.

[metalmatze]

YAY! 🎉 🚀
Soooo happy this has the same impact for Parca.

[thorfour]

LGTM

[metalmatze]

So this is the biggest change I would still propose to this.
Given that we built this nested series>profiles>samples construct earlier on, but then use it here for just a bit of reusing labels, I'd propose to flatten this earlier already.
Basically, the Ingest takes in []normalizedSamples and we only have one loop to iterate over.