Allow single fields in fields and _source parameters

[pquentin]

This was initially reported in elastic/elasticsearch-py#2347

The _source body field used to work but was broken in https://github.com/elastic/elasticsearch-specification/pull/1027/files#diff-4114e335296798822015a064d77c5c6d14c797a89c76a0d08b63dcf5d4028651.

Based on https://www.elastic.co/guide/en/elasticsearch/reference/current/search-fields.html, I fixed _source, fields and docvalue_fields.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
msearch	🟢	17/17	16/16
search	🔴	1519/1538	1517/1520

You can validate these APIs yourself by using the make validate target.

[JoshMock]

LGTM

[github-actions]

The backport to 7.17 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-7.17 7.17
# Navigate to the new working tree
cd .worktrees/backport-7.17
# Create a new branch
git switch --create backport-2317-to-7.17
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 9f40be8b64f80a8602c362aae25ff27be71e1b9a
# Push it to GitHub
git push --set-upstream origin backport-2317-to-7.17
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.17

Then, create a pull request where the base branch is 7.17 and the compare/head branch is backport-2317-to-7.17.

[github-actions]

The backport to 8.10 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-8.10 8.10
# Navigate to the new working tree
cd .worktrees/backport-8.10
# Create a new branch
git switch --create backport-2317-to-8.10
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 9f40be8b64f80a8602c362aae25ff27be71e1b9a
# Push it to GitHub
git push --set-upstream origin backport-2317-to-8.10
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.10

Then, create a pull request where the base branch is 8.10 and the compare/head branch is backport-2317-to-8.10.

[github-actions]

The backport to 8.11 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-8.11 8.11
# Navigate to the new working tree
cd .worktrees/backport-8.11
# Create a new branch
git switch --create backport-2317-to-8.11
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 9f40be8b64f80a8602c362aae25ff27be71e1b9a
# Push it to GitHub
git push --set-upstream origin backport-2317-to-8.11
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.11

Then, create a pull request where the base branch is 8.11 and the compare/head branch is backport-2317-to-8.11.

[pquentin]

Backports:

• 7.17: [7.17] Allow single fields in fields and _source parameters #2332
• 8.10: [8.10] Allow single fields in fields and _source parameters #2331
• 8.11: [8.11] Allow single fields in fields and _source parameters #2330

[pquentin]

Since this is a breaking change for typed clients and too late for 8.11, I'm reverting the backports (but not the change in main):

• 7.17: Revert "[7.17] Allow single fields in fields and _source parameters" #2340
• 8.10: Revert "[8.10] Allow single fields in fields and _source parameters" #2339
• 811: Revert "[8.11] Allow single fields in fields and _source parameters (#2317)" #2338