Restrict /flush and /shutdown ingester endpoints to POST method only

[pracucci]

As discussed in https://github.com/cortexproject/cortex/pull/3228#discussion_r493859743, we should restrict /flush and /shutdown ingester endpoints to POST HTTP method only.

When doing it we should also:

1. Make it clear in the / page that they're POST only (because if you will click these links from the browser, it will show a 405 error)
2. Change tools/migrate-ingester-statefulsets.sh because calls GET /shutdown, but then we would have to use something different than wget (and curl is not available in Cortex images)

[roidelapluie]

Should we add a button on the page instead of the link?

[pstibrany]

Button sounds good to me. It may need some changes to “IndexPage” code.

[pracucci]

Change tools/migrate-ingester-statefulsets.sh because calls GET /shutdown, but then we would have to use something different than wget (and curl is not available in Cortex images)

I think I had my mind turned off when I wrote this. We don't run the script from the Cortex image. We can just replace it with curl -X POST.

[pstibrany]

I think I had my mind turned off when I wrote this. We don't run the script from the Cortex image. We can just replace it with curl -X POST.

We can run curl from machine that runs the script, but then we need to deal with background port forwarding and possible port collisions. When I tried that, port forward sometimes took longer to setup, so curl failed. In the end I chose to run “wget” from within the container instead, which doesn’t have these problems.