[ASM] Error metric for unknown operator errors (#6465)

When we experience an "unknown matcher operator" error in the WAF, we
should not increase the _dd.appsec.event_rules.error_count metric in the
span.

## Summary of changes

## Reason for change

## Implementation details

## Test coverage

## Other details
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tracer/src/Datadog.Trace/AppSec/Coordinator/SecurityCoordinator.Reporter.cs

@@ -92,8 +92,11 @@ internal static void ReportWafInitInfoOnce(Security security, Span span)
             security.WafInitResult.Reported = true;
             span.Context.TraceContext?.SetSamplingPriority(SamplingPriorityValues.UserKeep, SamplingMechanism.Asm);
             span.SetMetric(Metrics.AppSecWafInitRulesLoaded, security.WafInitResult.LoadedRules);
-            span.SetMetric(Metrics.AppSecWafInitRulesErrorCount, security.WafInitResult.FailedToLoadRules);
-            if (security.WafInitResult.HasErrors && !Security.HasOnlyUnknownMatcherErrors(security.WafInitResult.Errors))
+            bool onlyUnknownMatcherErrors = Security.HasOnlyUnknownMatcherErrors(security.WafInitResult.Errors);
+
+            // If there are only unknown matcher errors, we don't want to report the failures
+            span.SetMetric(Metrics.AppSecWafInitRulesErrorCount, onlyUnknownMatcherErrors ? 0 : security.WafInitResult.FailedToLoadRules);
+            if (security.WafInitResult.HasErrors && !onlyUnknownMatcherErrors)
             {
                 span.SetTag(Tags.AppSecWafInitRuleErrors, security.WafInitResult.ErrorMessage);
             }