prometheus: Refactor getAttrs

[pellared]

Addresses #5932 (comment)

I feel that getAttr was doing too much and having:

keys, values := getAttrs(dp.Attributes)
keys = append(keys, kv.keys...)
values = append(values, kv.vals...)
// ...
keys, values := getAttrs(*res.Set())

is more readable than:

keys, values := getAttrs(dp.Attributes, ks, vs, resourceKV)
// ...
keys, values := getAttrs(*res.Set(), [2]string{}, [2]string{}, keyVals{})

Benchmarks results just in case to minimize the possibility of accidental introduction of a performance overhead:

$ benchstat old.txt new.txt 
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/exporters/prometheus
cpu: Intel(R) Core(TM) i9-10885H CPU @ 2.40GHz
                │   old.txt    │               new.txt               │
                │    sec/op    │    sec/op     vs base               │
Collect1-16       29.77µ ± 11%   30.33µ ± 10%       ~ (p=0.529 n=10)
Collect10-16      80.58µ ±  7%   77.93µ ± 15%       ~ (p=0.315 n=10)
Collect100-16     528.5µ ±  7%   511.2µ ±  2%  -3.28% (p=0.015 n=10)
Collect1000-16    3.179m ±  6%   3.344m ± 15%  +5.19% (p=0.003 n=10)
Collect10000-16   31.77m ±  2%   33.14m ±  7%  +4.34% (p=0.004 n=10)
geomean           662.9µ         668.9µ        +0.90%

                │   old.txt    │               new.txt               │
                │     B/op     │     B/op      vs base               │
Collect1-16       36.52Ki ± 0%   36.59Ki ± 0%  +0.17% (p=0.000 n=10)
Collect10-16      64.58Ki ± 0%   64.64Ki ± 0%  +0.09% (p=0.000 n=10)
Collect100-16     349.3Ki ± 0%   349.4Ki ± 0%  +0.03% (p=0.000 n=10)
Collect1000-16    3.163Mi ± 0%   3.163Mi ± 0%       ~ (p=0.247 n=10)
Collect10000-16   31.05Mi ± 0%   31.06Mi ± 0%  +0.02% (p=0.009 n=10)
geomean           610.6Ki        611.0Ki       +0.06%

                │   old.txt   │              new.txt               │
                │  allocs/op  │  allocs/op   vs base               │
Collect1-16        70.00 ± 0%    72.00 ± 0%  +2.86% (p=0.000 n=10)
Collect10-16       396.0 ± 0%    398.0 ± 0%  +0.51% (p=0.000 n=10)
Collect100-16     3.661k ± 0%   3.663k ± 0%  +0.05% (p=0.000 n=10)
Collect1000-16    36.15k ± 0%   36.15k ± 0%  +0.01% (p=0.000 n=10)
Collect10000-16   361.4k ± 0%   361.5k ± 0%  +0.03% (p=0.009 n=10)
geomean           4.212k        4.241k       +0.68%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.6%. Comparing base (7fd5942) to head (631cc0b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5937     +/-   ##
=======================================
- Coverage   84.6%   84.6%   -0.1%     
=======================================
  Files        272     272             
  Lines      22854   22858      +4     
=======================================
  Hits       19339   19339             
- Misses      3171    3175      +4     
  Partials     344     344             

see 2 files with indirect coverage changes

[pellared]

@dashpole, PTAL