Add headers for WebSocket

[shokada]

When Loki Canary connects to a gateway, the following error occurred.

failed to connect to ws://xxxx/api/prom/tail?query=%7Bstream%3D%22stdout%22%2Cname%3D%22loki-canary%22%7D with err websocket: bad handshake

Loki Canary opens a WebSocket connection to Loki, so the gateway needs to send a request including the Upgrade and Connection headers.
http://nginx.org/en/docs/http/websocket.html

And, since /loki/api/v1/tail and /api/prom/tail are handled in Querier module instead of QueryFrontend module, the gateway needs to send a request to Querier.

[codecov-io]

Codecov Report

Merging #1689 into master will decrease coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1689      +/-   ##
==========================================
- Coverage   62.09%   61.98%   -0.11%     
==========================================
  Files         111      111              
  Lines        8391     8391              
==========================================
- Hits         5210     5201       -9     
- Misses       2788     2793       +5     
- Partials      393      397       +4

Impacted Files	Coverage Δ
pkg/promtail/targets/tailer.go	73.56% <0%> (-2.3%)	⬇️
pkg/promtail/targets/filetarget.go	68.71% <0%> (-1.85%)	⬇️
pkg/ingester/transfer.go	65% <0%> (-1.43%)	⬇️
pkg/logql/evaluator.go	91.83% <0%> (-1.03%)	⬇️

[cyriltovena]

@shokada Thanks for the contribution, we don't use this gateway ourselves, we have our own. Have you tested this in your cluster ?

[shokada]

@cyriltovena Yes, I tested this in my cluster. It seems to work.

[owen-d]

After our discussion, LGTM pending removal of the OrgID headers.

[slim-bean]

I think this looks good! I will take a final look in the next day or so but we should be able to get this merged, thanks @shokada

[cyriltovena]

LGTM