Added experimental features flag

[metonymic-smokey]

Signed-off-by: metonymic-smokey ahuja.aditi@gmail.com

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Fixes #4022
This PR adds an --enable-feature flag to query, similar to Prom's --enable-features flag.
The features that can be currently turned on are enabling negative offset and @ modifier.

Verification

[metonymic-smokey]

@yeya24 @bwplotka this is the first draft of the PR.
Please let me know if I should add this change to the changelog and if there is something I could improve here.
Thanks!

[yeya24]

Looks good! Please update the changelog and add a doc about feature flags. You can add this to different components. https://github.com/thanos-io/thanos/tree/main/docs/components

[yeya24]

This pr looks good to me once the docs CI issue is fixed. @thanos-io/thanos-maintainers Does this experimental flags design look good?

[bwplotka]

Minor nits only, otherwise LGTM, thanks!

[bwplotka]

Tiny nit, otherwise LGTM! Thanks 💪🏽

[metonymic-smokey]

Running make docs locally, should have this docs test passing by then.

[bwplotka]

LGTM, make docs to be run

[yeya24]

LGTM

[zhongpengqun]

spec:
  containers:
  - args:
    - query
    - --http-address=0.0.0.0:9090
    - --store=rdoss-thanos-sidecar.svc.eng.xxxx.com:10901
    - --store=rdoss-thanos-store.svc.eng.xxxx.com:10901
    - --query.lookback-delta=2h
    - --enable-feature=promql-at-modifier
    image: xxxx.com/dockerhub-proxy-cache/thanosio/thanos:v0.24.0-rc.1
    imagePullPolicy: IfNotPresent
    livenessProbe:
      failureThreshold: 3
      httpGet:
        path: /-/healthy
        port: 9090
        scheme: HTTP
      periodSeconds: 10
      successThreshold: 1
      timeoutSeconds: 1

Hi, i want to add the parameter --enable-feature=promql-at-modifier
so i run command kubectl edit pod/rdoss-thanos-query-675d49c7b8-fqc7g
and add the parameter to test if is it OK,

but it denied to save which means the flag is illegal.

I have no idea why, please help me.