Scheduler: throw UnsupportedOperationException if appropriate

[mkouba]

• also refactor the implementation and introduce BaseScheduler
• resolves Scheduler: clarify what happens if scheduler is not started but method declared on Scheduler API is invoked #44101

[manovotn]

Added just one tiny comment with optional change.
I don't insist on it being modified before merging if you don't think it's needed.

[quarkus-bot]

Status for workflow Quarkus CI

This is the status report for running Quarkus CI on commit e94ff3d.

✅ 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/opentelemetry/deployment

✖ io.quarkus.opentelemetry.deployment.metrics.HttpServerMetricsTest.collectsHttpRouteFromEndAttributes - History

• Assertion condition defined as a Lambda expression in io.quarkus.opentelemetry.deployment.common.exporter.InMemoryMetricExporter expected: <true> but was: <false> within 5 seconds. - org.awaitility.core.ConditionTimeoutException

org.awaitility.core.ConditionTimeoutException: Assertion condition defined as a Lambda expression in io.quarkus.opentelemetry.deployment.common.exporter.InMemoryMetricExporter expected: <true> but was: <false> within 5 seconds.
	at org.awaitility.core.ConditionAwaiter.await(ConditionAwaiter.java:167)
	at org.awaitility.core.AssertionCondition.await(AssertionCondition.java:119)
	at org.awaitility.core.AssertionCondition.await(AssertionCondition.java:31)
	at org.awaitility.core.ConditionFactory.until(ConditionFactory.java:1006)
	at org.awaitility.core.ConditionFactory.untilAsserted(ConditionFactory.java:790)
	at io.quarkus.opentelemetry.deployment.common.exporter.InMemoryMetricExporter.assertCountPointsAtLeast(InMemoryMetricExporter.java:131)
	at io.quarkus.opentelemetry.deployment.common.exporter.InMemoryMetricExporter_ClientProxy.assertCountPointsAtLeast(Unknown Source)

• Assertion condition defined as a Lambda expression in io.quarkus.opentelemetry.deployment.common.exporter.InMemoryMetricExporter expected: <true> but was: <false> within 5 seconds. - org.awaitility.core.ConditionTimeoutException

org.awaitility.core.ConditionTimeoutException: Assertion condition defined as a Lambda expression in io.quarkus.opentelemetry.deployment.common.exporter.InMemoryMetricExporter expected: <true> but was: <false> within 5 seconds.
	at org.awaitility.core.ConditionAwaiter.await(ConditionAwaiter.java:167)
	at org.awaitility.core.AssertionCondition.await(AssertionCondition.java:119)
	at org.awaitility.core.AssertionCondition.await(AssertionCondition.java:31)
	at org.awaitility.core.ConditionFactory.until(ConditionFactory.java:1006)
	at org.awaitility.core.ConditionFactory.untilAsserted(ConditionFactory.java:790)
	at io.quarkus.opentelemetry.deployment.common.exporter.InMemoryMetricExporter.assertCountPointsAtLeast(InMemoryMetricExporter.java:131)
	at io.quarkus.opentelemetry.deployment.common.exporter.InMemoryMetricExporter_ClientProxy.assertCountPointsAtLeast(Unknown Source)

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⚙️ Native Tests - HTTP

📦 integration-tests/rest-client-reactive

✖ io.quarkus.it.rest.client.BasicTestIT.shouldCreateClientSpans - History

• expected: <1> but was: <2> - org.opentest4j.AssertionFailedError

org.opentest4j.AssertionFailedError: expected: <1> but was: <2>
	at io.quarkus.it.rest.client.BasicTest.shouldCreateClientSpans(BasicTest.java:216)
	at java.base/java.lang.reflect.Method.invoke(Method.java:569)
	at io.quarkus.test.junit.QuarkusTestExtension.interceptTestMethod(QuarkusTestExtension.java:810)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)