refactor(sdk-metrics): fix eslint warning

```
/home/runner/work/opentelemetry-js/opentelemetry-js/packages/sdk-metrics/src/state/DeltaMetricProcessor.ts
  96:32  warning  Forbidden non-null assertion  @typescript-eslint/no-non-null-assertion
```

The assertion was intentional, base on the `.has()` check with the
same inputs immediately prior. The surrounding code has the same
patterns and used the eslint magic comment to disable the warning,
this one was just missed.

Ref open-telemetry#5365

Author: chancancode

packages/sdk-metrics/src/state/DeltaMetricProcessor.ts

@@ -93,6 +93,8 @@ export class DeltaMetricProcessor<T extends Maybe<Accumulation>> {
             attributes = this._overflowAttributes;
             hashCode = this._overflowHashCode;
             if (this._cumulativeMemoStorage.has(attributes, hashCode)) {
+              // has() returned true, previous is present.
+              // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
               const previous = this._cumulativeMemoStorage.get(
                 attributes,
                 hashCode
@@ -103,7 +105,7 @@ export class DeltaMetricProcessor<T extends Maybe<Accumulation>> {
         }
         // Merge with uncollected active delta.
         if (this._activeCollectionStorage.has(attributes, hashCode)) {
-          // has() returned true, previous is present.
+          // has() returned true, active is present.
           // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
           const active = this._activeCollectionStorage.get(
             attributes,