Make Micrometer test dependencies optional

To make sure io.micrometer.core.instrument and
io.micrometer.observation packages are marked as
optional in the OSGi manifest.

Fixes #1140

Author: acogoluegnes

pom.xml

@@ -805,12 +805,14 @@
       <artifactId>micrometer-tracing-integration-test</artifactId>
       <version>${micrometer-tracing-test.version}</version>
       <scope>test</scope>
+      <optional>true</optional>
     </dependency>
     <dependency>
       <groupId>io.micrometer</groupId>
       <artifactId>micrometer-docs-generator</artifactId>
       <version>${micrometer-docs-generator.version}</version>
       <scope>test</scope>
+      <optional>true</optional>
     </dependency>
 
   </dependencies>