Restrict push CI to main branch

[anuraaga]

Currently, CI push events run on any branch except for some

https://github.com/open-telemetry/opentelemetry-python/blob/main/.github/workflows/test_0.yml#L8

This means that they run on the pushes to someone's fork as part of a PR. This doubles CI load with no reason and can increase notification spam for flaky builds. I couldn't find any obvious branch that needs CI events except for main, does it make sense to set CI to only run for main push to avoid forks?

[emdneto]

That makes sense. We skip otelbot and release branches to prevent CI failures during releases caused by the cross-SHA dependency between -core and -contrib. This way, if someone needs to push a commit to a release branch, it won’t impact the PR CI.