x-pack/filebeat/input/cel: fix handling of infinite values in rates

The protobuf.Value.AsInterface method encodes +Inf as "Infinity" so
ensure that we accept that string.

CHANGELOG.next.asciidoc

@@ -277,6 +277,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 - Add support for base64-encoded HMAC headers to HTTP Endpoint. {pull}39655[39655]
 - Add user group membership support to Okta entity analytics provider. {issue}39814[39814] {pull}39815[39815]
 - Add request trace support for Okta and EntraID entity analytics providers. {pull}39821[39821]
+- Fix handling of infinite rate values in CEL rate limit handling logic. {pull}39940[39940]
 
 *Auditbeat*
 

x-pack/filebeat/input/cel/input.go

@@ -706,7 +706,7 @@ func getLimit(which string, rateLimit map[string]interface{}, log *logp.Logger)
 	case float64:
 		limit = rate.Limit(r)
 	case string:
-		if !strings.EqualFold(r, "inf") {
+		if !strings.EqualFold(strings.TrimPrefix(r, "+"), "inf") && !strings.EqualFold(strings.TrimPrefix(r, "+"), "infinity") {
 			log.Errorw("unexpected value returned for rate limit "+which, "value", r, "rate_limit", mapstr.M(rateLimit))
 			return limit, false
 		}