Merge pull request moby#4959 from profnandaa/doc-jaeger-on-windows

docs: add a note for opentel/jaeger on windows

Author: tonistiigi

README.md

@@ -800,6 +800,10 @@ export JAEGER_TRACE=0.0.0.0:6831
 # any buildctl command should be traced to http://127.0.0.1:16686/
 ```
 
+> On Windows, if you are running Jaeger outside of a container, [`jaeger-all-in-one.exe`](https://www.jaegertracing.io/docs/1.57/getting-started/#all-in-one),
+> set the environment variable `setx -m JAEGER_TRACE "0.0.0.0:6831"`,
+> restart `buildkitd` in a new terminal and the traces will be collected automatically.
+
 ## Running BuildKit without root privileges
 
 Please refer to [`docs/rootless.md`](docs/rootless.md).