[chore] Auto-populate release notes (#12644)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
This PR changes the release workflow to autofill the release notes from
`CHANGELOG.md` and `CHANGELOG-API.md` into the generated GH release.

It makes use od `awk` and `sed` to build the release notes step by step
from the changelog files.
The [default chloggen
template](https://github.com/open-telemetry/opentelemetry-go-build-tools/blob/c43cb0331ccdb9ff9b4a8862a3dce032147e615d/chloggen/internal/chlog/summary.tmpl)
was added and a `<!--preview-version-->` tag was added to easily filter
out the changelog of just the latest version.

<!-- Issue number if applicable -->
#### Link to tracking issue
Fixes #10191

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Tested on my fork.
Release with autofilled changelog:
https://github.com/mowies/opentelemetry-collector/releases/tag/v0.121.0
Workflow that did it:
https://github.com/mowies/opentelemetry-collector/actions/runs/13899615357/job/38888008499

<!--Describe the documentation added.-->
#### Documentation
The release checklist was updated accordingly.

<!--Please delete paragraphs that you did not use before submitting.-->

---------

Signed-off-by: Moritz Wiesinger <[email protected]>

Author: mowies

.chloggen/config.yaml

@@ -12,6 +12,8 @@ entries_dir: .chloggen
 # (Optional) Default: .chloggen/TEMPLATE.yaml
 template_yaml: .chloggen/TEMPLATE.yaml
 
+summary_template: .chloggen/summary.tmpl
+
 # The CHANGELOG file or files to which 'chloggen update' will write new entries
 # (Optional) Default filename: CHANGELOG.md
 change_logs:

.chloggen/summary.tmpl

@@ -0,0 +1,70 @@
+{{- define "entry" -}}
+- `{{ .Component }}`: {{ .Note }} (
+{{- range $i, $issue := .Issues }}
+{{- if $i }}, {{ end -}}
+#{{ $issue }}
+{{- end -}}
+)
+
+{{- if .SubText }}
+{{ .SubText | indent 2 }}
+{{- end }}
+{{- end }}
+## {{ .Version }}
+
+{{- if .BreakingChanges }}
+
+### 🛑 Breaking changes 🛑
+
+{{- range $i, $change := .BreakingChanges }}
+{{- if eq $i 0}}
+{{end}}
+{{ template "entry" $change }}
+{{- end }}
+{{- end }}
+
+{{- if .Deprecations }}
+
+### 🚩 Deprecations 🚩
+
+{{- range $i, $change := .Deprecations }}
+{{- if eq $i 0}}
+{{end}}
+{{ template "entry" $change }}
+{{- end }}
+{{- end }}
+
+{{- if .NewComponents }}
+
+### 🚀 New components 🚀
+
+{{- range $i, $change := .NewComponents }}
+{{- if eq $i 0}}
+{{end}}
+{{ template "entry" $change }}
+{{- end }}
+{{- end }}
+
+{{- if .Enhancements }}
+
+### 💡 Enhancements 💡
+
+{{- range $i, $change := .Enhancements }}
+{{- if eq $i 0}}
+{{end}}
+{{ template "entry" $change }}
+{{- end }}
+{{- end }}
+
+{{- if .BugFixes }}
+
+### 🧰 Bug fixes 🧰
+
+{{- range $i, $change := .BugFixes }}
+{{- if eq $i 0}}
+{{end}}
+{{ template "entry" $change }}
+{{- end }}
+{{- end }}
+
+<!-- previous-version -->

.github/workflows/sourcecode-release.yaml

@@ -13,9 +13,26 @@ jobs:
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
           fetch-depth: 0
+
+      - name: Prepare release notes
+        run: |
+          touch release-notes.md
+          echo "### Images and binaries here: https://github.com/open-telemetry/opentelemetry-collector-releases/releases/tag/${{ github.ref_name }}" >> release-notes.md
+          echo "" >> release-notes.md
+          echo "## End User Changelog" >> release-notes.md
+
+          awk '/<!-- next version -->/,/<!-- previous-version -->/' CHANGELOG.md > tmp-chlog.md # select changelog of latest version only
+          sed '1,3d' tmp-chlog.md >> release-notes.md # delete first 3 lines of file
+
+          echo "" >> release-notes.md
+          echo "## API Changelog" >> release-notes.md
+
+          awk '/<!-- next version -->/,/<!-- previous-version -->/' CHANGELOG-API.md > tmp-chlog-api.md # select changelog of latest version only
+          sed '1,3d' tmp-chlog-api.md >> release-notes.md # delete first 3 lines of file
+
       - name: Create Github Release
         run: |
-          gh release create ${{ github.ref_name }} -t ${{ github.ref_name }} -n "### Images and binaries here: https://github.com/open-telemetry/opentelemetry-collector-releases/releases/tag/${{ github.ref_name }}"
+          gh release create ${{ github.ref_name }} -t ${{ github.ref_name }} -F release-notes.md
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       - uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1

docs/release.md

@@ -63,7 +63,7 @@ Before the release, make sure there are no open release blockers in [core](https
 
 6. Wait for the new tag build to pass successfully.
 
-7. A new `v0.85.0` source code release should be automatically created on Github by now. Edit it and use the contents from the CHANGELOG.md and CHANGELOG-API.md as the release's description.
+7. A new `v0.85.0` source code release should be automatically created on Github by now. Its description should already contain the corresponding CHANGELOG.md and CHANGELOG-API.md contents.
 
 ## Releasing opentelemetry-collector-contrib