[example] add Spark framework example

dd-trace-examples/rest-spark/.gitignore

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+# Compiled class file
+*.class
+
+# Log file
+*.log
+
+# BlueJ files
+*.ctxt
+
+# Mobile Tools for Java (J2ME)
+.mtj.tmp/
+
+# Package Files #
+*.jar
+*.war
+*.ear
+*.zip
+*.tar.gz
+*.rar
+
+# virtual machine crash logs, see http://www.java.com/en/download/help/error_hotspot.xml
+hs_err_pid*
+
+# IDE
+.idea/

dd-trace-examples/rest-spark/build.gradle

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+group 'me.palazzetti'
+version '0.1.0-SNAPSHOT'
+
+apply plugin: 'java'
+
+sourceCompatibility = 1.8
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    compile fileTree(dir: 'libs', include: ['*.jar'])
+    compile 'com.sparkjava:spark-core:2.6.0'
+    compile 'org.mongodb:mongodb-driver:3.4.2'
+    compile 'io.opentracing:opentracing-api:0.30.0'
+    compile 'io.opentracing:opentracing-util:0.30.0'
+    compile 'io.opentracing.contrib:opentracing-mongo-driver:0.0.2'
+
+    testCompile group: 'junit', name: 'junit', version: '4.12'
+}

dd-trace-examples/rest-spark/settings.gradle

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+rootProject.name = 'rest-spark'