Change expectedSymbols to 2048

[SungJin1212]

This PR increases expectedSymbols to 2048 to reduce slice reallocations. The current value of 20 is very small in the production level.
As shown in the attached chart, after applying the change to 2048 (around 19:30), I have observed reduced allocation spikes.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[CharlieTLe]

Thanks, I wonder why it was set to 20 initially.

[friedrichg]

Suggested change

	* [CHANGE] Distributor: Increase the default capacity of `expectedSymbols` from 20 to 2048 in the PRW2 path to reduce slice reallocations. #7397
	* [CHANGE] Distributor: Increase the default capacity of `expectedSymbols` from 20 to 2048 in the PRW2 path to reduce slice reallocations. #7398

[friedrichg]

Ultimately, this is about labels. if your requests have more labels, you will need this.

what do you think about something like this?

var symbolsHighWaterMark atomic.Int64

  func init() {
        symbolsHighWaterMark.Store(int64(expectedSymbols))
  }

  func ReuseWriteRequestV2(req *PreallocWriteRequestV2) {
        // Update high watermark, capped to prevent outliers from inflating all allocations.
        if n := int64(cap(req.Symbols)); n > symbolsHighWaterMark.Load() && n <= 8192 {
                symbolsHighWaterMark.Store(n)
        }
        // ... existing cleanup code ...
  }

  writeRequestPoolV2 = sync.Pool{
        New: func() any {
                return &PreallocWriteRequestV2{
                        WriteRequestV2: WriteRequestV2{
                                Symbols: make([]string, 0, symbolsHighWaterMark.Load()),
                        },
                }
        },
  }

Starts at expectedSymbols (20), grows to match real usage, caps at 8192 (128KB per pooled object) to prevent runaway allocation from outlier requests.

[SungJin1212]

Sound goods, I introduced an adaptive way. Can you take a look?
I think the initial 20 is small, so set it to 128.

[SungJin1212]

@CharlieTLe
I think when I introduced PRW2 initially, I was unable to set the value carefully.

[friedrichg]

LGTM