[config/configretry] Allow zero multiplier and arbitrary randomization factor (#9235)

**Description:** 

Partial revert of #9091 to unblock
open-telemetry/opentelemetry-collector-contrib/pull/30167

This does not mean that usage of a zero multiplier or a randomization
factor outside of [0,1] is blessed upon by Collector maintainers, just
that we need to unblock the release :)

---------

Signed-off-by: Alex Boten <[email protected]>
Co-authored-by: Alex Boten <[email protected]>

Author: mx-psi

.chloggen/addretrysetvalidation.yaml

@@ -9,5 +9,8 @@ component: "exporterhelper"
 # A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
 note: "Add RetrySettings validation function"
 
+subtext: |
+  Validate that time.Duration, multiplier values in configretry are non-negative, and randomization_factor is between 0 and 1
+
 # One or more tracking issues or pull requests related to the change
 issues: [9089]

config/configretry/backoff.go

@@ -52,8 +52,8 @@ func (bs *BackOffConfig) Validate() error {
 	if bs.RandomizationFactor < 0 || bs.RandomizationFactor > 1 {
 		return errors.New("'randomization_factor' must be within [0, 1]")
 	}
-	if bs.Multiplier <= 0 {
-		return errors.New("'multiplier' must be positive")
+	if bs.Multiplier < 0 {
+		return errors.New("'multiplier' must be non-negative")
 	}
 	if bs.MaxInterval < 0 {
 		return errors.New("'max_interval' must be non-negative")

config/configretry/backoff_test.go

@@ -43,10 +43,17 @@ func TestInvalidRandomizationFactor(t *testing.T) {
 func TestInvalidMultiplier(t *testing.T) {
 	cfg := NewDefaultBackOffConfig()
 	assert.NoError(t, cfg.Validate())
-	cfg.Multiplier = 0
+	cfg.Multiplier = -1
 	assert.Error(t, cfg.Validate())
 }
 
+func TestZeroMultiplierIsValid(t *testing.T) {
+	cfg := NewDefaultBackOffConfig()
+	assert.NoError(t, cfg.Validate())
+	cfg.Multiplier = 0
+	assert.NoError(t, cfg.Validate())
+}
+
 func TestInvalidMaxInterval(t *testing.T) {
 	cfg := NewDefaultBackOffConfig()
 	assert.NoError(t, cfg.Validate())