Add flatten-maven-plugin for Sonatype Central Portal (#3825)

Same as openzipkin/zipkin-reporter-java#284

Signed-off-by: Adrian Cole <adrian@tetrate.io>

Author: codefromthecrypt

.gitignore

@@ -26,3 +26,4 @@ _site/
 # This project does not use Yarn but some developers may use it to e.g., start zipkin-lens dev server.
 # It doesn't hurt to just exclude it here.
 yarn.lock
+.flattened-pom.xml

pom.xml

@@ -92,6 +92,7 @@
     <license.skip>${skipTests}</license.skip>
 
     <build-helper-maven-plugin.version>3.6.1</build-helper-maven-plugin.version>
+    <flatten-maven-plugin.version>1.7.3</flatten-maven-plugin.version>
     <go-offline-maven-plugin.version>1.2.8</go-offline-maven-plugin.version>
     <!-- TODO: cleanup any redundant ignores now also in the 4.0 release (once final) -->
     <license-maven-plugin.version>5.0.0</license-maven-plugin.version>
@@ -327,6 +328,12 @@
           </configuration>
         </plugin>
 
+        <plugin>
+          <groupId>org.codehaus.mojo</groupId>
+          <artifactId>flatten-maven-plugin</artifactId>
+          <version>${flatten-maven-plugin.version}</version>
+        </plugin>
+
         <plugin>
           <groupId>org.sonatype.central</groupId>
           <artifactId>central-publishing-maven-plugin</artifactId>
@@ -666,6 +673,30 @@
       <id>release</id>
       <build>
         <plugins>
+          <plugin>
+            <groupId>org.codehaus.mojo</groupId>
+            <artifactId>flatten-maven-plugin</artifactId>
+            <configuration>
+              <flattenMode>ossrh</flattenMode>
+            </configuration>
+            <executions>
+              <execution>
+                <id>flatten</id>
+                <phase>process-resources</phase>
+                <goals>
+                  <goal>flatten</goal>
+                </goals>
+              </execution>
+              <execution>
+                <id>flatten.clean</id>
+                <phase>clean</phase>
+                <goals>
+                  <goal>clean</goal>
+                </goals>
+              </execution>
+            </executions>
+          </plugin>
+
           <plugin>
             <groupId>org.sonatype.central</groupId>
             <artifactId>central-publishing-maven-plugin</artifactId>