util: Pass limit to strutil.MergeSlices

[harry671003]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

MergeSlices and MergeUnsortedSlices now accepts a limit param which will make the merge faster when a lmit is passed.

Benchmark

(base) ➜  thanos git:(merge_slices_with_limit) ✗ benchstat old.txt new.txt
goos: darwin
goarch: arm64
pkg: github.com/thanos-io/thanos/pkg/strutil
cpu: Apple M1 Pro
                         │      old.txt      │                  new.txt                  │
                         │      sec/op       │      sec/op        vs base                │
MergeSlices/benchmark-10   0.00275000n ± 13%   0.00006540n ± 16%  -97.62% (p=0.000 n=10)

                         │  old.txt   │            new.txt             │
                         │    B/op    │    B/op     vs base            │
MergeSlices/benchmark-10   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

                         │  old.txt   │            new.txt             │
                         │ allocs/op  │ allocs/op   vs base            │
MergeSlices/benchmark-10   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

Verification

• Added unit tests
• Added benchmarks

[yeya24]

Looks great. Can you add a benchmark test case so that we can understand how much it improves?

[yeya24]

Thanks!