fix(ui/src/alerting): reduce points per check from 300 to 100

[desa]

This was done to reduce amount of data returned to the client. This is
problematic when there are a large number of series that get returned
from a query. Long term we will want to limit the topN tables so that
this is not an issue.

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass
• http/swagger.yml updated (if modified Go structs or API)
• Documentation updated or issue created (provide link to issue/pr)
• Signed CLA (if not already signed)

[drdelambre]

yolo 🍰 lgtm

[hoorayimhelping]

this is problematic when there are a large number of series that get returned
from a query

does this affect smaller queries or normal day-to-day operations? is there a way to ease this on for larger queries? like say, if the query has a certain number of results, use the lower value? is that overkill?

[desa]

This is only for the number of points per series that we display in graph when building a check. The main idea behind this feature is to give the user a window of data that they can view. If the user picked a 1m group by, they would see the last 100m of data. This is definitely not a long term solution and meant as a bandaid for an existing issue. Long term we'll want to limit the number of series that are returned to only, the top N.