Show the metadata tags in the flamegraph UI by default

[NHDaly]

Add an option to show the metadata tags in the flamegraph view, as root nodes, like this:

I don't love that the tags aren't labelled..... I've asked about that here:
google/pprof#811

I'm also not sure if tagroot= is the right way to expose this. Maybe it should be higher-level, like metadata::Bool and we just add this? Or groupby=["taskid", "threadid"], and we turn that into tagroots?

[codecov]

Codecov Report

❌ Patch coverage is 97.43590% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.01%. Comparing base (6e9f56c) to head (c2ec74d).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/PProf.jl	97.43%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
- Coverage   96.66%   96.01%   -0.66%     
==========================================
  Files           3        3              
  Lines         300      326      +26     
==========================================
+ Hits          290      313      +23     
- Misses         10       13       +3     

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