[meta] Update changelog for 7.8.1 / 7.9.0 / 6.8.11 / 6.8.12 releases

[jmlrt]

• add missing changes for 7.9.0 / 6.8.12 release about Helm 3 beta support
• add more details about an issue related to Logstash breaking change in 7.8.1 / 6.8.11

Related to #766 and #759

[ghost]

@jmlrt When I got to "update the changelog" step, I wasn't sure where to get the contents from. How do you compile the changelog?

[jmlrt]

How do you compile the changelog?

Well that's another manual step...

My current process is reviewing every commits on the released branch since the last released version.

The good thing is that we enforce squash and merge so one commit is always one PR now.

git lg 7.9 --since="$(git log -1 7.9.0 --pretty=%ad)" | cat
* acfe3cd - (HEAD -> 7.9, origin/7.9) Fix serviceAccount for APM server (#786) (Dat Tang 6 days ago)
* b8a1367 - [metricbeat] Add missing labels for deployment (#770) (Vadym Liubko 6 days ago)
* e1efa56 - [logstash] Fix headless service ports spec (#776) (Simon Gurcke 6 days ago)
* fc0168c - Remove duplicate "initialDelaySeconds" field (#763) (Thomas Decaux 6 days ago)
* dcc1350 - Missing deletion of "elastic-certificate-crt" (#752) (AhmedSamirAhmed 6 days ago)
* 6d68ec5 - Fix typo in FAQ (#744) (SlavaSubotskiy 6 days ago)
* 49b2346 - [helm] bump helm version to 2.16.10 (#797) (Julien Mailleret 6 days ago)
* e1b0ad9 - [release] bump 7.9 branch to 7.9.1-SNAPSHOT (#791) (Yuriy 7 days ago)
* ccd2e6c - [meta] drop gke 1.14 tests (#798) (Julien Mailleret 7 days ago)
* 6cfbe00 - (tag: 7.9.0) 7.9.0 release (#788) (Yuriy 2 weeks ago)

Then I copy/paste a section from the previous release and update it manually with the content of the relevant commits.

The current format of the CHANGELOG.md is:

• commits/PRs related to feature/fix for one specific product are going to a table under the product sub-section
  • if a commit/PR is adding a feature/fix for more than one product, I duplicate the line under every related product sub-section
  • PR titles can be cleaned and edited in the table to be more explicit
• changes not related to products feature/fix (example product or helm version bump, CI changes, process documentation...) can be listed if relevant (less details needed, author of the change is often mentionned but not necessarily the exact PR of the change (there is no real rule for that).
• finally I run docker run --rm -it -v $(pwd)/CHANGELOG.md:/usr/src/CHANGELOG.md jorgeandrada/doctoc to update the table of content.

I had a little python script to retrieve all merged PR already formated as a table (thanks to @Crazybus), but then it requires manual steps to reorder them and filter only what was merged or backported on the good branch, and since the last release (some old PR could be at the top of the list if someone recently commented it after merge...), edit the PR title if needed, filter relevant PRs and add theM to the relevant product sub-sections...

So unfortunately, it is currently faster to fill changelog manually by copy/pasting as long as I don't have time to update the script.

At some time we should update the CHANGELOG format to make it easier to update and create process to automate it more (maybe by using PR tags or gitmoji to generate it for exemple...).

For the BREAKING_CHANGES.md, this is a mix of Elasticsearch breaking change notes and Elasticsearch release highlights.

I use it to document breaking changes, and important changes that would require more details than a line in the changelog and don't have their place in the charts README documentation, known bugs and their workarounds, ...

I fill it after checking the details of all PRs in the changelog and recent issues/user feedbacks.

This is typically something that should stay manual and can't be automated IMO.