MeterTagsSupport: prevent null metric tag values

[adutra]

This situation can happen when ValueExpressionResolver.resolve() returns null. The fix is to return an empty string instead.

Fixes #47891

[quarkus-bot]

/cc @brunobat (micrometer), @ebullient (micrometer)

[quarkus-bot]

Status for workflow Quarkus CI

This is the status report for running Quarkus CI on commit 20c8bd0.

✅ The latest workflow run for the pull request has completed successfully.

It should be safe to merge provided you have a look at the other checks in the summary.

You can consult the Develocity build scans.

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Flaky tests - Develocity

⚙️ JVM Tests - JDK 17 Windows

📦 extensions/micrometer-opentelemetry/deployment

✖ io.quarkus.micrometer.opentelemetry.deployment.compatibility.MicrometerTimedInterceptorTest.testTimeMethod_Async - History

• Stream has no elements - java.lang.IllegalArgumentException

java.lang.IllegalArgumentException: Stream has no elements
	at io.quarkus.micrometer.opentelemetry.deployment.common.MetricDataFilter.lambda$lastReadingPointsSize$5(MetricDataFilter.java:219)
	at java.base/java.util.Optional.orElseThrow(Optional.java:403)
	at io.quarkus.micrometer.opentelemetry.deployment.common.MetricDataFilter.lastReadingPointsSize(MetricDataFilter.java:219)
	at io.quarkus.micrometer.opentelemetry.deployment.common.InMemoryMetricExporter.lambda$assertCountDataPointsAtLeastOrEqual$4(InMemoryMetricExporter.java:88)
	at org.awaitility.core.AssertionCondition.lambda$new$0(AssertionCondition.java:53)
	at org.awaitility.core.ConditionAwaiter$ConditionPoller.call(ConditionAwaiter.java:248)
	at org.awaitility.core.ConditionAwaiter$ConditionPoller.call(ConditionAwaiter.java:235)