Make micrometer-bom a java-platform so it only publishes a pom

The nebula publication was publishing source, javadoc, and binary jars along with the pom. The correct way to just publish a pom (BOM) in Gradle is to use the `java-platform` plugin. This requires that the project not be a `java` or `java-library` project.

Resolves #1618

build.gradle

@@ -27,8 +27,43 @@ allprojects {
 }
 
 subprojects {
-    apply plugin: 'java'
-    apply plugin: 'checkstyle'
+    if (project.name != 'micrometer-bom') {
+        apply plugin: 'java'
+        apply plugin: 'checkstyle'
+
+        tasks {
+            compileJava {
+                options.encoding = 'UTF-8'
+                // ensure Java 8 baseline is enforced for main source
+                if (JavaVersion.current().isJava9Compatible()) {
+                    options.compilerArgs.addAll(['--release', '8'])
+                }
+            }
+            compileTestJava {
+                options.encoding = 'UTF-8'
+                sourceCompatibility = JavaVersion.current()
+                targetCompatibility = JavaVersion.current()
+            }
+        }
+
+        //noinspection GroovyAssignabilityCheck
+        test {
+            // set heap size for the test JVM(s)
+            maxHeapSize = "1500m"
+
+            useJUnitPlatform()
+        }
+
+        checkstyle {
+            toolVersion = '8.23'
+            configFile = rootProject.file('config/checkstyle/checkstyle.xml')
+        }
+
+        license {
+            ext.year = Calendar.getInstance().get(Calendar.YEAR)
+            skipExistingHeaders = true
+        }
+    }
 
     dependencyLocking {
         lockAllConfigurations()
@@ -49,21 +84,6 @@ subprojects {
         }
     }
 
-    tasks {
-        compileJava {
-            options.encoding = 'UTF-8'
-            // ensure Java 8 baseline is enforced for main source
-            if (JavaVersion.current().isJava9Compatible()) {
-                options.compilerArgs.addAll(['--release', '8'])
-            }
-        }
-        compileTestJava {
-            options.encoding = 'UTF-8'
-            sourceCompatibility = JavaVersion.current()
-            targetCompatibility = JavaVersion.current()
-        }
-    }
-
     if(!['samples', 'benchmarks'].find{project.name.contains(it)}) {
         apply plugin: 'io.spring.publishing'
 
@@ -87,26 +107,8 @@ subprojects {
         mavenLocal()
     }
 
-    checkstyle {
-        toolVersion = '8.23'
-        configFile = rootProject.file('config/checkstyle/checkstyle.xml')
-    }
-
     def check = tasks.findByName('check')
     if (check) project.rootProject.tasks.releaseCheck.dependsOn check
-
-    //noinspection GroovyAssignabilityCheck
-    test {
-        // set heap size for the test JVM(s)
-        maxHeapSize = "1500m"
-
-        useJUnitPlatform()
-    }
-
-    license {
-        ext.year = Calendar.getInstance().get(Calendar.YEAR)
-        skipExistingHeaders = true
-    }
 }
 
 wrapper {

micrometer-bom/build.gradle

@@ -1,4 +1,5 @@
 plugins {
+    id 'java-platform'
     id 'io.spring.dependency-management' version '1.0.8.RELEASE'
 }
 
@@ -17,3 +18,11 @@ dependencyManagement {
         }
     }
 }
+
+publishing {
+    publications {
+        nebula(MavenPublication) {
+            from components.javaPlatform
+        }
+    }
+}