telemetry: add acceptedCharCount to UserTriggerDecision STE (#5846)

JB's PR: aws/aws-toolkit-jetbrains#5008

## Problem


## Solution


---

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License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: andrewyuq

packages/core/src/codewhisperer/util/telemetryHelper.ts

@@ -332,6 +332,7 @@ export class TelemetryHelper {
                         suggestionReferenceCount: referenceCount,
                         generatedLine: generatedLines,
                         numberOfRecommendations: suggestionCount,
+                        acceptedCharacterCount: acceptedRecommendationContent.length,
                     },
                 },
             })