contrib/go-redis/redis.v8: optimize BeforeProcess and BeforeProcessPipeline

[johejo]

This change includes the following three optimizations.

1. The previous implementation split the command with strings.Split, but only
   the 0th element is actually needed as a tag for span.
   Therefore, use strings.IndexByte to find the first space and slice the raw
   command string.

2. Replace the length derivation of args with the counting of the number of
   spaces.

3. The number of ddtrace.StartSpanOptions is often pre-determined,
   so make it in advance to reduce allocation.

Benchmark

func BenchmarkBeforeProcess(b *testing.B) {
	opts := &redis.Options{Addr: "127.0.0.1:6379"}
	client := redis.NewClient(opts)
	ctx := context.Background()
	cmd := redis.NewStringCmd(ctx, "set", "key", "value", 99)
	cfg := new(clientConfig)
	defaults(cfg)
	ddh := &datadogHook{params: &params{additionalTags: additionalTagOptions(client), config: cfg}}
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_, err := ddh.BeforeProcess(ctx, cmd)
		if err != nil {
			b.Fatal(err)
		}
	}
}

Result

name             old time/op    new time/op    delta
BeforeProcess-4     568ns ± 1%     447ns ± 1%  -21.30%  (p=0.000 n=9+10)

name             old alloc/op   new alloc/op   delta
BeforeProcess-4      640B ± 0%      512B ± 0%  -20.00%  (p=0.000 n=10+10)

name             old allocs/op  new allocs/op  delta
BeforeProcess-4      15.0 ± 0%      13.0 ± 0%  -13.33%  (p=0.000 n=10+10)

[gbbr]

This is nice, thanks! Just one nit, otherwise all good by me.

[gbbr]

Real nice! Thanks for posting the benchmarks.