Provide starters for micrometer-tracing

Getting all the dependencies right for various tracing options is no easy task. We should provide starters to make that easier for users.

For example, using Brave + Zipkin, we need:

<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-actuator</artifactId>
</dependency>
<dependency>
	<groupId>io.micrometer</groupId>
	<artifactId>micrometer-tracing</artifactId>
</dependency>
<dependency>
	<groupId>io.micrometer</groupId>
	<artifactId>micrometer-tracing-bridge-brave</artifactId>
</dependency>
<dependency>
	<groupId>io.zipkin.reporter2</groupId>
	<artifactId>zipkin-reporter-brave</artifactId>
</dependency>

and, if there’s no RestTemplate or WebClient, we additionally need:

<dependency>
	<groupId>io.zipkin.reporter2</groupId>
	<artifactId>zipkin-sender-urlconnection</artifactId>
</dependency>

[wilkinsona]

To help us in the future when we’re trying to remember why we reached the decision, here’s a brief summary of what we considered.

The combinations that can be used are listed in the documentation:

• OpenTelemetry With Zipkin
  • io.micrometer:micrometer-tracing-bridge-otel
  • io.opentelemetry:opentelemetry-exporter-zipkin
• OpenTelemetry With Wavefront
  • io.micrometer:micrometer-tracing-bridge-otel
  • io.micrometer:micrometer-tracing-reporter-wavefront
• OpenZipkin Brave With Zipkin
  • io.micrometer:micrometer-tracing-bridge-brave
  • io.zipkin.reporter2:zipkin-reporter-brave
• OpenZipkin Brave With Wavefront
  • io.micrometer:micrometer-tracing-bridge-brave
  • io.micrometer:micrometer-tracing-reporter-wavefront

Starters are intended to help people to get started but we have four different combinations here. We considered offering a starter for each combination but concluded that offering four starters would be counter productive.

We considered a more opinionated arrangement that would have been somewhat like spring-boot-starter-web and it using spring-boot-starter-tomcat by default. The goal would have been to have a Zipkin or Wavefront starter that uses, say, OpenTelemetry by default and to be able to exclude the OpenTelemetry starter in favour of a Brave starter. Unfortunately, this doesn’t work as the Zipkin dependencies different depending upon whether you’re using OpenTelemetry or Brave.

In the end, given that each combination is only two dependencies, we concluded that it’s easiest to declare those dependencies directly. This isn’t that different to the existing arrangement for metrics where a dependency on the appropriate registry implementation (Prometheus, Datadog, Wavefront, and so on) has to be declared. We can revisit this decision in the future if the code and module structure of the dependencies changes.

[marcingrzejszczak]

Sure, I’m working on some samples over here https://github.com/micrometer-metrics/micrometer-samples/tree/migrate-sleuth-samples . You can check e.g. mvc and resttemplate to see web related examples (not all of them are already prepared)