Add changelog entry

Author: ocelotl

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- Do not produce any metric output without metric data
+  ([#3335](https://github.com/open-telemetry/opentelemetry-python/pull/3335))
 - Modify Prometheus exporter to translate non-monotonic Sums into Gauges
   ([#3306](https://github.com/open-telemetry/opentelemetry-python/pull/3306))