[breaking] Reduce allocations in LineToEvents by returning a single event instead of an slice

[pedro-stanaka]

Description

The current implementation of the LineToEvents method in pkg/line/line.go returns multiple events, which results in a lot of allocations in the statsd-exporter. We propose changing this method to return a single event instead.
On the same refactoring step, we should introduce a "exploder" or "sampling" layer (or helper method on the event) which will keep the same behavior as we have now.
This is a breaking change on purpose which will force client projects using this repository as a library, to adapt to the new method signature.

Here is an example profile with samples using sampling rate, where you can see it takes more than 21% of total memory used:

Goals

1. Reduce Allocations: By returning a single event, we aim to reduce the number of allocations and improve performance.
2. Enforce Build Breaks: Changing the method signature will force dependent projects to update their code, ensuring compatibility with the new implementation.
3. Introduce New Layer: Push the decision about what to do with Count or SamplingRate to another layer of the exporter.

Proposed Solution

1. Change Method Signature: Modify the LineToEvents method to return a single event.
2. Update Event Structure: Include Count or SamplingRate in the event structure.
3. Introduce New Layer: Implement a new layer in the exporter to handle the decision-making process for Count or SamplingRate.

Repro script

	dogClient, err := statsd.New(
		opts.statsdServer,
		statsd.WithoutTelemetry(),
		statsd.WithNamespace("flood_statsd"),
		statsd.WithTags([]string{"pod_name:" + os.Getenv("POD_NAME")}),
		statsd.WithoutClientSideAggregation(),
	)
	if err != nil {
		level.Error(logger).Log("msg", "Error creating dogClient client", "err", err)
		os.Exit(1)
	}

	samplesSent := 0

	start := time.Now()
	for {
		randomInt := rand.Int63n(opts.cardinalityLimit)
		err := dogClient.Count("sample_counter", 1, []string{"mybad_label:co_" + strconv.FormatInt(randomInt, 10)}, 0.01)
		if err != nil {
			level.Error(logger).Log("msg", "Error sending metric", "err", err)
		}
		_ = dogClient.Distribution("synthetic", float64(randomInt), []string{"mybad_label:co" + strconv.FormatInt(randomInt, 10)}, 0.001)
		_ = dogClient.Timing(
			"some_timing",
			time.Duration(rand.Intn(3000))*time.Millisecond,
			[]string{"cardinality:" + strconv.FormatInt(randomInt, 10)},
			0.01,
		)
		_ = dogClient.Gauge("some_gauge", float64(randomInt), []string{"mybad_label:co" + strconv.FormatInt(randomInt, 10)}, 0.01)

		_ = dogClient.Distribution("heavily_sampled_distribution", float64(randomInt), []string{"mybad_label:co" + strconv.FormatInt(randomInt, 10)}, 0.001)

		// control the rate of the synthetic metrics
		samplesSent += 4
		if samplesSent >= int(opts.samplesPerSec) {
			elapsed := time.Since(start)
			if elapsed < time.Second {
				level.Info(logger).Log("msg", "Sleeping for", "duration", time.Second-elapsed)
				time.Sleep(time.Second - elapsed)
			}
			level.Info(logger).Log("msg", "Sending", "samples", samplesSent, "elapsed", elapsed)
			samplesSent = 0
			start = time.Now()
		}
	}

[matthiasr]

Thank you! This topic also intersects with #558, where we unpack dogstatsd extended aggregations into individual events. Instead, it might be better to keep the values as a list, without duplicating the event metadata around them.

I want this to be an explicitly breaking change, because otherwise adding a sampling rate field would be a silently breaking change, where downstream processing of the events doesn't obviously break, but produces incorrect results by not taking the sampling rate into account