Speed-up flame graph generation by skipping unneeded work.

[ghemawat]

Previously, we used to call report.TextItems() during flame graph generation just so we could get a hand on the legend to print in the profile details box. All the work done by TextItems() to produce a trimmed graph was discarded.

This change separates out the legend generation into a separate routine so that we can avoid doing the unnecessary work.

Benchmark result:

name      old time/op  new time/op  delta
Flame-12   6.10s ± 3%   0.39s ± 4%  -93.59%  (p=0.000 n=10+10)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.02%. Comparing base (0ed6a68) to head (a99a299).
Report is 30 commits behind head on main.

Files	Patch %	Lines
internal/report/report.go	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #881      +/-   ##
==========================================
+ Coverage   66.86%   67.02%   +0.16%     
==========================================
  Files          44       44              
  Lines        9824     9796      -28     
==========================================
- Hits         6569     6566       -3     
+ Misses       2794     2778      -16     
+ Partials      461      452       -9     

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