Is your feature request related to a problem? Please describe.

We are currently trying to get loki-canary to work with the Docker log driver in a Swarm environnement. But it seems they are simply not compatible, as loki-canary is incapable of fetching the correct labels (there are "labelname" and "labelvalue" configurations already, but one is missing for "stream").

Loki-canary expects a "stream" label (stdout/stderr), while the Docker log driver publishes this under the "source" label. Looking at the source code, it looks like both are hardcoded.

Example:

loki-canary:
  query={stream="stdout",swarm_service="monitoring_loki-canary"} => KO (Empty)

manually:
  query={source="stdout",swarm_service="monitoring_loki-canary"} => Success

Source:

loki-canary (reader.go #L106)

Docker log driver (loki.go #L60)

Describe the solution you'd like

Both projects should allow customization, starting with loki-canary to mitigate this very case.
But I think that the Docker log driver should too, in order to play nice with other implementations based on "stream" that might exist (canary, generic grafana dashboards, etc)

Describe alternatives you've considered

There are no known workarounds.
We could use promtail of course, but it would defeat the purpose of loki-canary, since we would use a different log transmission medium for the rest of the swarm (Docker log driver).

Additional context
none