If the other Prometheus has an external label that matches that of
the Prometheus being read from, then we need to remove that matcher
from the request as it's not actually stored in the database - it's
only added for alerts, federation and on the output of the remote read
endpoint.
Instead we check for that label being empty, in case there is a time
series with a different label value for that external label.

Signed-off-by: Goutham Veeramachaneni <cs14btech11014@iith.ac.in>

Signed-off-by: Goutham Veeramachaneni <cs14btech11014@iith.ac.in>

Apparently, a decision was made at some point to only use the v prefix
in tags and similar contexts where other things can appear. There was a
vote to stick to that decision. For more information, read
https://github.com/prometheus/prometheus/pull/2743.

This reverts commit 5405a472.

Sadly, this is urgently required.

This got dropped in the cherrypicking.

It now follows the policy of only documenting user-relevant changes.