contributing: add jaeger override to support tracing feature

as a trace collector needs to be set up for one to consume the distr
tracing functionality, here I add a docker-compose override[1] that's
supposed to be merged together with our base `docker-compose.yml` to add
`jaeger` and tie `web` with it, so that one can visualize the traces in
the Jaeger UI.

[1]: https://docs.docker.com/compose/extends/

Signed-off-by: Ciro S. Costa <cscosta@pivotal.io>

CONTRIBUTING.md

@@ -211,6 +211,25 @@ After this is done, the final step is to connect your IDE to the debugger with t
 For GoLand you can do so by going to Run | Edit Configurations… | + | Go Remote and fill in the parameters.
 
 
+### Trying out distributed tracing with Jaeger
+
+Under `./hack`, a docker-compose override file named `jaeger.yml` provides the
+essentials to get [Jaeger] running alongside the other components, as well tying
+Concourse to it through the right environment variables.
+
+[Jaeger]: https://jaegertracing.io
+
+To leverage that extension, run `docker-compose up` specifying where all the
+yaml files are:
+
+```sh
+$ docker-compose \
+  -f ./docker-compose.yml \
+  -f ./hack/overrides/jaeger.yml \
+  up -d
+```
+
+
 ### Connecting to Postgres
 
 If you want to poke around the database, you can connect to the `db` node using

hack/overrides/jaeger.yml

@@ -0,0 +1,30 @@
+# jaeger.yml - a docker-compose override that adds 'jaeger to the stack.
+#
+# once running, head to `localhost:25000` to get access to the Jaeger UI.
+#
+# ref: https://www.jaegertracing.io/
+# ref: https://docs.docker.com/compose/extends/
+#
+version: '3'
+
+services:
+  web:
+    environment:
+      CONCOURSE_TRACING_JAEGER_COMPONENT: atc
+      CONCOURSE_TRACING_JAEGER_ENDPOINT: http://jaeger:14268/api/traces
+
+  jaeger:
+    image: jaegertracing/all-in-one:1.14
+    command:
+      - --sampling.strategies-file=/etc/jaeger/sampling_strategies.json
+      - --log-level=debug
+    environment:
+      COLLECTOR_ZIPKIN_HTTP_PORT: 9411
+    ports:
+      - '14268:14268'
+      - '25000:16686'
+      - '5775:5775/udp'
+      - '5778:5778'
+      - '6831:6831/udp'
+      - '6832:6832/udp'
+      - '9411:9411'