Start adding a demo for opencensus-contrib-log-correlation-stackdriver (Java).

This commit adds a demo for log correlation using OpenCensus, Logback, and
Stackdriver.  It is very similar to the demo that was originally contained in
census-instrumentation/opencensus-java#1212.

The demo can be run with the following command in the
`java/log_correlation/stackdriver/logback/` directory:

`./gradlew clean installDist && ./build/install/logback/bin/OpenCensusTraceLoggingEnhancerDemo`

Author: sebright2

java/log_correlation/stackdriver/logback/README.md

@@ -0,0 +1,13 @@
+# OpenCensus/Logback/Stackdriver Log Correlation Demo
+
+An application that demonstrates log correlation in Stackdriver with
+`opencensus-contrib-log-correlation-stackdriver`. The application contains SLF4J log statements and
+OpenCensus tracing instrumentation. It configures logging with a Logback XML configuration and
+exports logs using the
+[Stackdriver Logging Logback appender](https://cloud.google.com/logging/docs/setup/java#logback_appender).
+It also exports traces using `opencensus-exporter-trace-stackdriver`, so that Stackdriver can show
+the log entries associated with each trace.
+
+The application requires a Google Cloud project with Stackdriver Logging enabled. It can be run with
+
+`./gradlew clean installDist && ./build/install/logback/bin/OpenCensusTraceLoggingEnhancerDemo`

java/log_correlation/stackdriver/logback/build.gradle

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+description = 'OpenCensus/Logback/Stackdriver Log Correlation Demo'
+
+apply plugin: 'java'
+
+repositories {
+    mavenCentral()
+}
+
+group = "io.opencensus"
+version = "0.16.0-SNAPSHOT"
+
+def opencensusVersion = "0.15.0"
+
+tasks.withType(JavaCompile) {
+    sourceCompatibility = '1.7'
+    targetCompatibility = '1.7'
+}
+
+dependencies {
+    compile "io.opencensus:opencensus-api:${opencensusVersion}",
+            "io.opencensus:opencensus-exporter-trace-stackdriver:${opencensusVersion}",
+            "org.slf4j:slf4j-api:1.7.25"
+
+    runtime "io.opencensus:opencensus-contrib-log-correlation-stackdriver:${opencensusVersion}",
+            "io.opencensus:opencensus-impl-lite:${opencensusVersion}",
+            "com.google.cloud:google-cloud-logging-logback:0.48.0-alpha"
+}
+
+apply plugin: 'application'
+
+startScripts.enabled = false
+
+task openCensusLogCorrelationStackdriverDemo(type: CreateStartScripts) {
+    mainClassName =
+        'io.opencensus.demo.logcorrelation.stackdriver.logback.OpenCensusTraceLoggingEnhancerDemo'
+    applicationName = 'OpenCensusTraceLoggingEnhancerDemo'
+    outputDir = new File(project.buildDir, 'tmp')
+    classpath = jar.outputs.files + project.configurations.runtime
+}
+
+applicationDistribution.into('bin') {
+    from(openCensusLogCorrelationStackdriverDemo)
+    fileMode = 0755
+}

java/log_correlation/stackdriver/logback/gradle/wrapper/gradle-wrapper.jar

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+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.9-bin.zip
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists