Reduce allocation in discovery by ~30%

[thampiotr]

PR Description

This is a low hanging fruit fix to reduce allocation in discovery components by around 30%. More work may still be required in the future.

Which issue(s) this PR fixes

Notes to the Reviewer

thampiotr@MacWork ~/w/alloy (thampiotr/reduce-disco-allocation)> go test -count 5 -bench=Benchmark_ToAlloyTargets -benchmem ./internal/component/discovery/ | tee after.txt; benchstat before.txt after.txt
goos: darwin
goarch: arm64
pkg: github.com/grafana/alloy/internal/component/discovery
cpu: Apple M2
Benchmark_ToAlloyTargets-8   	     150	   7549967 ns/op	12768249 B/op	   40433 allocs/op
Benchmark_ToAlloyTargets-8   	     169	   7257841 ns/op	12767441 B/op	   40430 allocs/op
Benchmark_ToAlloyTargets-8   	     171	   7026276 ns/op	12767394 B/op	   40430 allocs/op
Benchmark_ToAlloyTargets-8   	     170	   7060700 ns/op	12767377 B/op	   40430 allocs/op
Benchmark_ToAlloyTargets-8   	     170	   7034392 ns/op	12767427 B/op	   40430 allocs/op
PASS
ok  	github.com/grafana/alloy/internal/component/discovery	15.349s
goos: darwin
goarch: arm64
pkg: github.com/grafana/alloy/internal/component/discovery
cpu: Apple M2
                  │  before.txt  │              after.txt              │
                  │    sec/op    │    sec/op     vs base               │
_ToAlloyTargets-8   9.679m ± ∞ ¹   7.061m ± ∞ ¹  -27.05% (p=0.008 n=5)
¹ need >= 6 samples for confidence interval at level 0.95

                  │  before.txt   │              after.txt               │
                  │     B/op      │     B/op       vs base               │
_ToAlloyTargets-8   18.24Mi ± ∞ ¹   12.18Mi ± ∞ ¹  -33.24% (p=0.008 n=5)
¹ need >= 6 samples for confidence interval at level 0.95

                  │  before.txt  │              after.txt              │
                  │  allocs/op   │  allocs/op    vs base               │
_ToAlloyTargets-8   60.45k ± ∞ ¹   40.43k ± ∞ ¹  -33.12% (p=0.008 n=5)

And the before:

cat before.txt
goos: darwin
goarch: arm64
pkg: github.com/grafana/alloy/internal/component/discovery
cpu: Apple M2
Benchmark_ToAlloyTargets-8   	     123	  10320177 ns/op	19125568 B/op	   60453 allocs/op
Benchmark_ToAlloyTargets-8   	     122	   9597422 ns/op	19124639 B/op	   60450 allocs/op
Benchmark_ToAlloyTargets-8   	     123	   9639463 ns/op	19125584 B/op	   60454 allocs/op
Benchmark_ToAlloyTargets-8   	     123	   9684174 ns/op	19124906 B/op	   60451 allocs/op
Benchmark_ToAlloyTargets-8   	     120	   9678773 ns/op	19125101 B/op	   60452 allocs/op
PASS
ok  	github.com/grafana/alloy/internal/component/discovery	16.564s

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

[wildum]

nice catch