increase and simplify backoff

The current backoffDuration is designed to deal with optimistic
concurrency control, which is not what we're trying to do. More
importantly, we're only doing a single retry right now if the first
attempt failed, so backoffDuration() call always returns 100ms +/- some
jitter. This means that the moment there is an upstream issue (e.g. S3
rate limit hit), we immediately spam the upstream service with a second
request which is usually counter-productive.

This patch simply sleeps for rand(0, profilePeriod). The profile period
by default is 60s, so on average we'll sleep for 30s, i.e. even space
out our attempt to upload the profile.

Attached to the PR are two illustrations that show the old and new
behavior in a simple simulation.

profiler/upload.go

@@ -10,7 +10,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"math"
 	"math/rand"
 	"mime/multipart"
 	"net/http"
@@ -25,18 +24,6 @@ const maxRetries = 2
 var errOldAgent = errors.New("Datadog Agent is not accepting profiles. Agent-based profiling deployments " +
 	"require Datadog Agent >= 7.20")
 
-// backoffDuration calculates the backoff duration given an attempt number and max duration
-func backoffDuration(attempt int, max time.Duration) time.Duration {
-	// https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/
-	if attempt == 0 {
-		return 0
-	}
-	maxPow := float64(max / 100 * time.Millisecond)
-	pow := math.Min(math.Pow(2, float64(attempt)), maxPow)
-	ns := int64(float64(100*time.Millisecond) * pow)
-	return time.Duration(rand.Int63n(ns))
-}
-
 // upload tries to upload a batch of profiles. It has retry and backoff mechanisms.
 func (p *profiler) upload(bat batch) error {
 	statsd := p.cfg.statsd
@@ -45,7 +32,7 @@ func (p *profiler) upload(bat batch) error {
 		err = p.doRequest(bat)
 		if rerr, ok := err.(*retriableError); ok {
 			statsd.Count("datadog.profiler.go.upload_retry", 1, nil, 1)
-			wait := backoffDuration(i+1, p.cfg.cpuDuration)
+			wait := time.Duration(rand.Int63n(p.cfg.period.Nanoseconds()))
 			log.Error("Uploading profile failed: %v. Trying again in %s...", rerr, wait)
 			time.Sleep(wait)
 			continue