Remove default value for OtlpMetricsProperties.url

[izeye]

Unlike Micrometer, Spring Boot's Micrometer integration doesn't support the OTEL_EXPORTER_OTLP_ENDPOINT and OTEL_EXPORTER_OTLP_METRICS_ENDPOINT environment variables due to its default value.

This PR removes the default value for the OtlpMetricsProperties.url property to try to align with Micrometer.

[mhalbritter]

This would also align with OtlpLoggingProperties and OtlpTracingProperties, which have no default for their endpoint property.

It should also be transparent to the user, as the OtlpMetricsPropertiesConfigAdapter calls io.micrometer.registry.otlp.OtlpConfig#url when the property is null and this returns http://localhost:4318/v1/metrics as a default.

[mhalbritter]

This library has been added for mocking environment variables. But on the GitHub page it says:

From JDK16 onwards, there are deeper restrictions on the ability to use reflection. [...] Consequently, this library now uses bytebuddy to enable the interception of calls for reading environment variables. This might interact with your chosen version of Mockito or other libraries.

I wanted to know the team's opinion if we want to have that library for tests or if we want to ditch the tests (whenPropertiesUrlIsNotSetAndOtelExporterOtlpEndpointIsSetAdapterUrlUsesIt and whenPropertiesUrlIsNotSetAndOtelExporterOtlpMetricsEndpointIsSetAdapterUrlUsesIt).

[nosan]

It seems like these tests verify the Micrometer OtlpConfig implementation rather than the OtlpMetricsPropertiesConfigAdapter.

I believe OtlpMetricsPropertiesConfigAdapter tests can be a little updated to ensure that
adapter delegates to OtlpConfig.url() when the URL is not set.

@Test
void whenPropertiesUrlIsNotSetAdapterUrlReturnsDefault() {
	assertThat(this.properties.getUrl()).isNull();
	assertThat(createAdapter().url()).isEqualTo("http://localhost:4318/v1/metrics");
}

@Test
void whenPropertiesUrlIsNotSetThanUseOtlpConfigUrlAsFallback() {
	assertThat(this.properties.getUrl()).isNull();
	OtlpMetricsPropertiesConfigAdapter adapter = spy(createAdapter());
	when(adapter.get("management.otlp.metrics.export.url")).thenReturn("https://my-endpoint/v1/metrics");
	assertThat(adapter.url()).isEqualTo("https://my-endpoint/v1/metrics");
}

In that case, uk.org.webcompere:system-stubs-jupiter:2.1.7 is unnecessary.

[wilkinsona]

I'm not in favor of trying to mock environment variables so +1 for @nosan's suggestion.

More generally, where we need to test environment variables, we try to allow the Map to be injected for testing purposes. If third-party code doesn't allow that then I'd prefer that we accept that it can't be tested rather than relying on reflection to hack into System.getenv.

[mhalbritter]

Thanks @izeye !