Create a request attribute (RA) to grab that portion of the URL. During creation there's a checkbox that allows the RA to access unmasked data. This process is similar to what you've already done for your Business Transaction RA.
Then create a calculated service metric (settings > server-side service monitoring > calculated service metrics) with a condition where your request attribute exists. Split by the request attribute itself.
At this point you have a metric split by each request attribute value (eg. ESC111511) and you can chart it using hte Metrics view and / or alert on it using "custom events for alerting" (settings > anomaly detection > custom events for alerting)