Use sync.Pool to reduce lengthReader allocations

[perhapsmaple]

Should fix #5756.

Tested allocations with this benchmark:

func BenchmarkServer_LengthReader(b *testing.B) {
	req := httptest.NewRequest(http.MethodGet, "/", nil)

	for i := 0; i < b.N; i++ {
		var bodyReader *lengthReader
		for j := 0; j < 10000; j++ {
			if req.Body != nil {
				// For testing without sync.Pool
				// bodyReader = &lengthReader{Source: req.Body}
				bodyReader = GetLengthReader(req.Body)
				req.Body = bodyReader
			}
			PutLengthReader(bodyReader)
		}
	}
	b.ReportAllocs()
}

Without sync.Pool:
BenchmarkServer_LengthReader-12 1808 700019 ns/op 240003 B/op 10000 allocs/op

With sync.Pool
BenchmarkServer_LengthReader-12 9516 127160 ns/op 0 B/op 0 allocs/op

This is my first time writing Go code, tried my best to stick to the same style. Successfully passed unit tests locally.

[CLAassistant]

All committers have signed the CLA.

[francislavoie]

I think since logRequest may return early due to SkipLog, this may leak as-is. I think it needs to put in that case as well.

[perhapsmaple]

Hi @francislavoie, I have added the requested changes. Let me know if any other changes are required

[mholt]

Thanks for this optimization! It's relatively simple and I believe it is correct.

Good job on one of your first Go contributions :) Thanks for the benchmark too!