api: ignore /api/gnet proxy when gzipping responses (grafana#71437)

This PR adds /api/gnet to the list of ignored paths in the gzip middleware.

Without this, when gzip is enabled (`server.enable_gzip = true`), responses
from the gnet proxy are double compressed: once by grafana.com and once by
Grafana itself. With this change we only do one round of compression for these
endpoints.

To test this out, try a request like this with `server.enable_gzip = true`
(after setting `GCOM_TOKEN` to a valid grafana.com token; you may need to
change the 'bsull' slug, too):

    curl -v --user admin:admin \
        -H "X-Api-Key: $GCOM_TOKEN" \
        -H 'Accept-Encoding: gzip' \
        localhost:3000/api/gnet/instances/bsull/provisioned-plugins/grafana-ml-app | gzip -d

Note that there are two Content-Encoding: gzip headers before this PR, and
the output is still compressed even after the `gzip -d`. After this PR things
look as expected.

Author: sd2k

pkg/middleware/gziper.go

@@ -45,6 +45,7 @@ var gzipIgnoredPaths = []matcher{
 	prefix("/api/datasources"),
 	prefix("/api/plugins"),
 	prefix("/api/plugin-proxy/"),
+	prefix("/api/gnet/"), // Already gzipped by grafana.com.
 	prefix("/metrics"),
 	prefix("/api/live/ws"),   // WebSocket does not support gzip compression.
 	prefix("/api/live/push"), // WebSocket does not support gzip compression.