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 71.06%. Comparing base (727a5b3) to head (270f8e7).

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   71.04%   71.06%   +0.02%     
==========================================
  Files         201      201              
  Lines       14773    14799      +26     
==========================================
+ Hits        10495    10517      +22     
- Misses       3530     3534       +4     
  Partials      748      748              

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