chore: track frac search wait time in explain

[cheb0]

Description

For some search requests, trace file looks like that:

i.e. there are holes when we wait for wait group in Searcher. I decided to expand explain entry as follows:

          {
            "duration": "302.14541ms",
            "message": "store/Search",
            "children": [
              {
                "duration": "17.553µs",
                "message": "parse query"
              },
              {
                "duration": "301.703205ms",
                "message": "search iteratively"
              },
              {
                "duration": "928.536095ms",
                "message": "search iteratively (cpu time)"
              },
              {
                "duration": "265.08246ms",
                "message": "waiting goroutines (all cores)"
              }
            ]
          }

i.e. number waiting goroutines shows how much time we spent waiting and we could do (and probable did) other work. However, if we processing a single search request at a time, then this time is essentially wasted. Obviously, this metric is not absolutely honest, system might have been significantly overloaded.

Numbers track times across all cores and do look odd, but I didn't find any other better way. I suppose we could do that, because explain entries are not loaded to tracing storage and we do not visualize them anywhere.

---

• I have read and followed all requirements in CONTRIBUTING.md;
• I used LLM/AI assistance to make this pull request;

[codecov-commenter]

Codecov Report

❌ Patch coverage is 67.74194% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.40%. Comparing base (660acaf) to head (0c70ee4).

Files with missing lines	Patch %	Lines
fracmanager/searcher.go	66.66%	9 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #305      +/-   ##
==========================================
- Coverage   71.60%   71.40%   -0.21%     
==========================================
  Files         204      204              
  Lines       14737    14763      +26     
==========================================
- Hits        10553    10541      -12     
- Misses       3431     3470      +39     
+ Partials      753      752       -1     

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[github-actions]

🔴 Performance Degradation

Some benchmarks have degraded compared to the previous run.
Click on Show table button to see full list of degraded benchmarks.

Show table

Name	Previous	Current	Ratio	Verdict
FindSequence_Random/medium-4	660aca	cea676
	8453.30 MB/s	6996.09 MB/s	0.83	🔴
	121.10 ns/op	146.40 ns/op	1.21	🔴
Indexer-4	660aca	cea676
	544073502.00 B/op	644523841.00 B/op	1.18	🔴