[processor/resourcedetection] Improve performance on MergeResource and detectedResource

[constanca-m]

Description

This PR introduces a couple of changes, so the processor performs better. You can see the results here:

$ benchstat old.txt new.txt
goos: linux
goarch: amd64
pkg: github.com/open-telemetry/opentelemetry-collector-contrib/processor/resourcedetectionprocessor/internal
cpu: 11th Gen Intel(R) Core(TM) i7-11800H @ 2.30GHz
                                           │    old.txt     │                new.txt                │
                                           │     sec/op     │    sec/op      vs base                │
MergeResource/From_1_Into_0-16                 99.30n ± 13%    81.12n ±  8%  -18.31% (p=0.001 n=10)
MergeResource/From_1000_Into_0-16            4021.69µ ± 11%    44.97µ ±  5%  -98.88% (p=0.000 n=10)
MergeResource/From_1_Into_1_Match-16           10.76n ±  9%    14.80n ± 23%  +37.59% (p=0.000 n=10)
MergeResource/From_1000_Into_1000_Match-16     1.658m ± 14%    2.002m ±  5%  +20.74% (p=0.004 n=10)
MergeResource/From_1000_Into_1000_50Pct-16     3.354m ±  8%    2.357m ± 21%  -29.72% (p=0.000 n=10)
ResourceProvider/ReadOnly-16                 73.7850n ±  6%   0.5283n ±  6%  -99.28% (p=0.000 n=10)
ResourceProvider/Mixed_1Pct_Write-16          188.60n ± 10%    28.93n ±  9%  -84.66% (p=0.000 n=10)
ResourceProvider/Mixed_10Pct_Write-16          775.0n ±  5%    308.9n ±  5%  -60.14% (p=0.000 n=10)
geomean                                        4.747µ          1.023µ        -78.44%

                                           │    old.txt     │                new.txt                 │
                                           │      B/op      │     B/op      vs base                  │
MergeResource/From_1_Into_0-16                 48.00 ± 0%       48.00 ± 0%        ~ (p=1.000 n=10) ¹
MergeResource/From_1000_Into_0-16            84.22Ki ± 0%     47.62Ki ± 0%  -43.45% (p=0.000 n=10)
MergeResource/From_1_Into_1_Match-16           0.000 ± 0%       0.000 ± 0%        ~ (p=1.000 n=10) ¹
MergeResource/From_1000_Into_1000_Match-16     0.000 ± 0%       0.000 ± 0%        ~ (p=1.000 n=10) ¹
MergeResource/From_1000_Into_1000_50Pct-16   7.935Ki ± 0%     7.912Ki ± 0%   -0.29% (p=0.011 n=10)
geomean                                                   ²                 -10.83%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean

                                           │    old.txt    │               new.txt                │
                                           │   allocs/op   │  allocs/op   vs base                 │
MergeResource/From_1_Into_0-16                2.000 ± 0%      2.000 ± 0%       ~ (p=1.000 n=10) ¹
MergeResource/From_1000_Into_0-16            1.011k ± 0%     1.001k ± 0%  -0.99% (p=0.000 n=10)
MergeResource/From_1_Into_1_Match-16          0.000 ± 0%      0.000 ± 0%       ~ (p=1.000 n=10) ¹
MergeResource/From_1000_Into_1000_Match-16    0.000 ± 0%      0.000 ± 0%       ~ (p=1.000 n=10) ¹
MergeResource/From_1000_Into_1000_50Pct-16    500.0 ± 0%      500.0 ± 0%       ~ (p=1.000 n=10) ¹
geomean                                                  ²                -0.20%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean