Optimize (attribute.Set).Filter for no filtered case

[MrAlias]

When all elements of the Set are kept during a call to (*Set).Filter, don't allocate a new Set the dropped attributes slice. Instead return the immutable Set and nil.

To achieve this the functionality of filterSet is broken down into a more generic filteredToFront function.

Benchmarks

$ benchstat main MrAlias:zero-alloc-unfilterd-set
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/attribute
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
                                          │    old.txt    │               new.txt                │
                                          │    sec/op     │    sec/op     vs base                │
Filtering/NoFilter/Set.Filter-8              2.835n ±  2%   3.085n ±  1%   +8.82% (p=0.000 n=10)
Filtering/NoFilter/NewSetWithFiltered-8      1.563µ ±  7%   1.723µ ± 74%        ~ (p=0.072 n=10)
Filtering/NoFiltered/Set.Filter-8           2906.5n ± 27%   621.9n ±  4%  -78.60% (p=0.000 n=10)
Filtering/NoFiltered/NewSetWithFiltered-8    3.031µ ± 17%   3.231µ ± 33%        ~ (p=0.796 n=10)
Filtering/Filtered/Set.Filter-8              2.404µ ± 12%   2.240µ ± 17%        ~ (p=0.579 n=10)
Filtering/Filtered/NewSetWithFiltered-8      1.291µ ±  2%   1.276µ ±  2%   -1.16% (p=0.037 n=10)
Filtering/AllDropped/Set.Filter-8            2.111µ ± 37%   2.441µ ± 11%        ~ (p=0.190 n=10)
Filtering/AllDropped/NewSetWithFiltered-8    752.7n ±  3%   758.8n ±  3%        ~ (p=0.853 n=10)
geomean                                      813.8n         698.3n        -14.20%

                                          │    old.txt     │                  new.txt                  │
                                          │      B/op      │     B/op      vs base                     │
Filtering/NoFilter/Set.Filter-8               0.000 ± 0%       0.000 ± 0%         ~ (p=1.000 n=10) ¹
Filtering/NoFilter/NewSetWithFiltered-8     3.501Ki ± 0%     3.501Ki ± 0%         ~ (p=1.000 n=10) ¹
Filtering/NoFiltered/Set.Filter-8           5.250Ki ± 0%     0.000Ki ± 0%  -100.00% (p=0.000 n=10)
Filtering/NoFiltered/NewSetWithFiltered-8   3.501Ki ± 0%     3.501Ki ± 0%         ~ (p=1.000 n=10) ¹
Filtering/Filtered/Set.Filter-8             1.812Ki ± 0%     1.812Ki ± 0%         ~ (p=1.000 n=10) ¹
Filtering/Filtered/NewSetWithFiltered-8       64.00 ± 0%       64.00 ± 0%         ~ (p=1.000 n=10) ¹
Filtering/AllDropped/Set.Filter-8           1.750Ki ± 0%     1.750Ki ± 0%         ~ (p=1.000 n=10) ¹
Filtering/AllDropped/NewSetWithFiltered-8     0.000 ± 0%       0.000 ± 0%         ~ (p=1.000 n=10) ¹
geomean                                                  ²                 ?                       ² ³
¹ all samples are equal
² summaries must be >0 to compute geomean
³ ratios must be >0 to compute geomean

                                          │   old.txt    │                 new.txt                 │
                                          │  allocs/op   │ allocs/op   vs base                     │
Filtering/NoFilter/Set.Filter-8             0.000 ± 0%     0.000 ± 0%         ~ (p=1.000 n=10) ¹
Filtering/NoFilter/NewSetWithFiltered-8     2.000 ± 0%     2.000 ± 0%         ~ (p=1.000 n=10) ¹
Filtering/NoFiltered/Set.Filter-8           3.000 ± 0%     0.000 ± 0%  -100.00% (p=0.000 n=10)
Filtering/NoFiltered/NewSetWithFiltered-8   2.000 ± 0%     2.000 ± 0%         ~ (p=1.000 n=10) ¹
Filtering/Filtered/Set.Filter-8             2.000 ± 0%     2.000 ± 0%         ~ (p=1.000 n=10) ¹
Filtering/Filtered/NewSetWithFiltered-8     1.000 ± 0%     1.000 ± 0%         ~ (p=1.000 n=10) ¹
Filtering/AllDropped/Set.Filter-8           1.000 ± 0%     1.000 ± 0%         ~ (p=1.000 n=10) ¹
Filtering/AllDropped/NewSetWithFiltered-8   0.000 ± 0%     0.000 ± 0%         ~ (p=1.000 n=10) ¹
geomean                                                ²               ?                       ² ³
¹ all samples are equal
² summaries must be >0 to compute geomean
³ ratios must be >0 to compute geomean

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (133f943) 82.2% compared to head (7c84fa5) 82.3%.

❗ Current head 7c84fa5 differs from pull request most recent head c2152dc. Consider uploading reports for the commit c2152dc to get more accurate results

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4774   +/-   ##
=====================================
  Coverage   82.2%   82.3%           
=====================================
  Files        226     226           
  Lines      18381   18392   +11     
=====================================
+ Hits       15127   15148   +21     
+ Misses      2972    2963    -9     
+ Partials     282     281    -1     

Files	Coverage Δ
attribute/set.go	80.4% <100.0%> (+2.6%)	⬆️

... and 1 file with indirect coverage changes