refactor(sumologicexporter): optimize fields stringification

[pmalek-sumo]

Use byte slices instead of strings where possible and modify a "stringified field byte slice" in place to prevent unnecessary copies.

Additionally short circuit in fields.equals() when attribute maps have different length to prevent using cmp for comparison (which is relatively expensive).

$ go test -v -count 1 -run ^$ -bench BenchmarkFields -benchmem -count 1 . > old.txt
$ gco -
Previous HEAD position was 86d1d22 tests(sumologicexporter): add fields stringification benchmark
Switched to branch 'optimize-fields'
$ go test -v -count 1 -run ^$ -bench BenchmarkFields -benchmem -count 1 . > new.txt
$ benchcmp old.txt new.txt
benchcmp is deprecated in favor of benchstat: https://pkg.go.dev/golang.org/x/perf/cmd/benchstat
benchmark              old ns/op     new ns/op     delta
BenchmarkFields-16     997           638           -35.97%

benchmark              old allocs     new allocs     delta
BenchmarkFields-16     17             11             -35.29%

benchmark              old bytes     new bytes     delta
BenchmarkFields-16     464           368           -20.69%