Limit input size for Prometheus alert JSON payload

Merged Vitali Tatarintev requested to merge limit-input-size-for-prometheus-alert-payload into master

What does this MR do?

Implements https://gitlab.com/gitlab-org/gitlab/issues/14929

Limit input size to prevent possible timeouts while processing an alert payload.

The generic alert payload parser has a similar check here and there.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

  • ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~
  • The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
  • Security reports checked/validated by a reviewer from the AppSec team
Edited by Vitali Tatarintev