Add support BP_DATADOGTRACE_ENABLED

[vchrisb]

Updated dependencies:

• golang 1.16
• pack 0.20.0
• jam 0.14.3
• buildpack.toml api 0.6

In addition to service binding will the buildpack participate if BP_DATADOGTRACE_ENABLED is set.

[vchrisb]

still WIP

[vchrisb]

I've successfully tested the pull request with a spring sample application and do see the datadog trace agent:

Picked up JAVA_TOOL_OPTIONS: -Djava.security.properties=/layers/paketo-buildpacks_bellsoft-liberica/java-security-properties/java-security.properties -agentpath:/layers/paketo-buildpacks_bellsoft-liberica/jvmkill/jvmkill-1.16.0-RELEASE.so=printHeapHistogram=1 -javaagent:/layers/paketo-buildpacks_datadog-trace/datadog-trace-java/dd-java-agent.jar -XX:ActiveProcessorCount=6 -XX:MaxDirectMemorySize=10M -Xmx2490385K -XX:MaxMetaspaceSize=88998K -XX:ReservedCodeCacheSize=240M -Xss1M -Dorg.springframework.cloud.bindings.boot.enable=true
[dd.trace 2021-08-20 10:28:57:263 +0000] [dd-task-scheduler] INFO datadog.trace.agent.core.StatusLogger - DATADOG TRACER CONFIGURATION {"version":"0.85.0~2626c788b3","os_name":"Linux","os_version":"5.10.25-linuxkit","architecture":"amd64","lang":"jvm","lang_version":"11.0.12","jvm_vendor":"BellSoft","jvm_version":"11.0.12+7-LTS","java_class_version":"55.0","http_nonProxyHosts":"null","http_proxyHost":"null","enabled":true,"service":"org.springframework.boot.loader.JarLauncher","agent_url":"http://localhost:8126","agent_error":true,"debug":false,"analytics_enabled":false,"sampling_rules":[{},{}],"priority_sampling_enabled":true,"logs_correlation_enabled":true,"profiling_enabled":false,"appsec_enabled":false,"dd_version":"0.85.0~2626c788b3","health_checks_enabled":true,"configuration_file":"no config file present","runtime_id":"b376de41-fea9-4a47-b829-9ed5dd049fed","logging_settings":{"levelInBrackets":false,"dateTimeFormat":"'[dd.trace 'yyyy-MM-dd HH:mm:ss:SSS Z']'","logFile":"System.err","configurationFile":"simplelogger.properties","showShortLogName":false,"showDateTime":true,"showLogName":true,"showThreadName":true,"defaultLogLevel":"INFO","warnLevelString":"WARN","embedException":false}}

An image with these changes is available here: ghcr.io/vchrisb/datadog-trace-paketo-buildpack/datadog-trace-paketo-buildpack:1.0.3
Due to the lack of a datadog environment, I was not able to test it end-to-end.

Looking at the Datadog Java Agent docs, any configuration files are optional, hence the introduction of BP_DATADOGTRACE_ENABLED to make enablement easier.
I also set the datadog agent version explicitly to have a deterministic version in the buildpack.

@kevinfwl could you have a look at the proposed changes and test it with a Datadog environment? 💯