chore: fix error in metrics.ts related to this.#cacheMissWindows.delete(firstKey)

[schaeferka]

Describe what should be investigated or refactored

this.#cacheMissWindows.delete(firstKey) is generating an error related to it possibly being undefined. This should be investigated and fixed.

Links to any relevant code

    if (maxCacheMissWindows !== undefined && this.#cacheMissWindows.size >= maxCacheMissWindows) {
      const firstKey = this.#cacheMissWindows.keys().next().value;
      this.#cacheMissWindows.delete(firstKey);
      this.#gauges.get(this.#getMetricName(this.#metricNames.cacheMiss))?.remove({ window: firstKey });
    }

Additional context

Error message:
Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
Type 'undefined' is not assignable to type 'string'.ts(2345)