Querier: Endpoint (store) flag should ignore empty or obviously incorrect addresses

[matej-g]

Thanos, Prometheus and Golang version used: Thanos - latest main build

What happened:
When I provide empty or obviously wrong address to the store or endpoint flag, query still tries to contact the store to obtain metadata, which can never succeed, resulting in logs being spammed with:

level=warn ts=2021-11-23T13:21:35.047200231Z caller=endpointset.go:500 component=endpointset msg="update of node failed" err="getting metadata: fallback fetching info from : rpc error: code = Canceled desc = context canceled" address=

What you expected to happen:
Thanos should ignore empty and obviously incorrect store / endpoint address; I would expect to be warned it the logs about the fact.

How to reproduce it (as minimally and precisely as possible):
Build Thanos from latest main (or just use latest release) and run query with flags --endpoint= (empty) and --endpoint=abc (obviously wrong)

Anything else we need to know:
At minimum, we could check if the address is in valid format <host>:<port> (or, if the DNS prefix is used, validate that); otherwise refuse it and give warning.