feat(api)!: Fail log queries when executed on instant query endpoint

[chaudum]

What this PR does / why we need it

Background

A log selector expression is a LogQL expression that returns logs, in contrast to a sample expressions, which returns metrics (samples). The simplest form of log selector expressions are label matchers, e.g. {env="prod"}.

Change

This PR changes the behaviour of Loki so that the instant query endpoint /api/v1/query does not allow sending a log selector expression as query any more. Instead, it returns a status code 400 (Bad Request) with the error message "log queries are not supported as an instant query type, please change you query to a range query type".

Why
Previously this API endpoint allowed these types of log queries, but returned inconsistent results, which where a major cause for confusion. Returning a concise error helps the user understand that they likely selected the wrong query type in Grafana when executing the query.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[na--]

🎉 ❤️ (Not super familiar with these parts of Loki yet, so a second pair of eyes would be nice)

[slim-bean]

LGTM, with one request:

Please add some details to the upgrade guide for this, I don't think it would be commonly used but if it was, the workaround here would be to do a range query with limit 30

[chaudum]

Please add some details to the upgrade guide for this, I don't think it would be commonly used but if it was, the workaround here would be to do a range query with limit 30

Ok, will do that in a follow up PR.

My reason for not putting anything into the UPGRADE guide was, that I thought the upgrade notes were for operational changes.

But I agree, it note would be helpful.