fix(deps): update dependency @google-cloud/opentelemetry-cloud-monitoring-exporter to ^0.21.0

[renovate-bot]

This PR contains the following updates:

Package	Change	Age	Confidence
@google-cloud/opentelemetry-cloud-monitoring-exporter	^0.20.0 -> ^0.21.0

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Release Notes

GoogleCloudPlatform/opentelemetry-operations-js (@​google-cloud/opentelemetry-cloud-monitoring-exporter)

v0.21.0

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