thanos-query: Wrong dropping store, external labels are not unique

[IKSIN]

Thanos, Prometheus and Golang version used:

Version 0.8.0
Revision 7b60b21
Branch HEAD
BuildUser circleci@32eb15a80c51
BuildDate 20191010-17:26:35
GoVersion go1.13.1

Object Storage Provider:
n/a

What happened:
thanos-query does not distinguish same labels from sidecar and store. When we run one sidecar and one store query shows error dropping store, external labels are not unique on sidecar.

What you expected to happen:
Error will not appear

How to reproduce it (as minimally and precisely as possible):

1. Run thanos-sidecar with external labels and configured storage.
2. Run thanos-store
3. Run thanos-query with configured storages on sidecar and store.
4. Look at the stores UI in thanos-query

Full logs to relevant components:

Anything else we need to know:
After restart thanos-store when thanos-query already running the problem not reproduce. But wen we restart thanos-query we see dropping store, external labels are not unique again.

[IKSIN]

Duplicate: #1632