Bump natsio/prometheus-nats-exporter in /helm/charts/nats (#893)

Bumps natsio/prometheus-nats-exporter from 0.14.0 to 0.15.0.

---
updated-dependencies:
- dependency-name: natsio/prometheus-nats-exporter
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

helm/charts/nats/values.yaml

@@ -378,7 +378,7 @@ promExporter:
   enabled: false
   image:
     repository: natsio/prometheus-nats-exporter
-    tag: 0.14.0
+    tag: 0.15.0
     pullPolicy:
     registry: