Loki: Add gzip compression to resource calls

[svennergr]

What is this feature?

Loki returns gzip compressed responses in certain situations. This PR enables this compression in the Loki datasource.

Why do we need this feature?

Improved performance if e.g. label values get large.

Special notes for your reviewer:

To get Loki into a setup where compression is enabled:

1. Add the following lines to https://github.com/grafana/grafana/blob/main/devenv/docker/blocks/loki/loki-config.yaml:

frontend:
  compress_responses: true

This enables compression on responses which are larger than 1400 bytes.
2. Create a bunch of labels to get more than 1400 bytes by changing the https://github.com/grafana/grafana/blob/main/devenv/docker/blocks/loki/data/data.js file. E.g. change the main method to:

async function main() {
  for (let step = 0; step < 300000; step++) {

    const timestampMs = new Date().getTime();
    const item = getRandomLogItem(step + 1)
    await lokiSendLogLine(timestampMs, JSON.stringify(item), {[`test${step}`]: step, place:'moon', source: 'data', instance: 'server\\1', job: '"grafana/data"'});
    // lokiSendLogLine(timestampMs, logFmtLine(item), {place:'luna', source: 'data', instance: 'server\\2', job: '"grafana/data"'});
  }
}

3. Run make devenv sources=loki
4. Go to explore, select Loki, select Query Builder, open the labels select-component while your browser's dev-tools are opened. Observe that the response to this request is now gzip-compressed:
   

[matyax]

The changes looks good. Just wanted to leave a note that I'm trying to get more involved in go code.

[grafanabot]

Drone build failed: https://drone.grafana.net/grafana/grafana-enterprise/42698

[gabor]

LGTM!

[grafanabot]

Drone build failed: https://drone.grafana.net/grafana/grafana-enterprise/42820