Add support for custom labels

[chaudum]

Passing a Label object as fifth argument to the Config constructor allows you to override the built-in labels by custom ones.

The custom labels map requires at least the format key, which needs to hold one of the supported log formats. The instance key will override the otherwise automatically generated instance label.

Example:

const labels = loki.Labels({
  "format": ["logfmt"], // must contain at least one of the supported log formats
  "os": ["linux"],
  "cluster": ["k3d", "minikube"],
  "namespace": ["loki-prod", "loki-dev"],
  "container": ["distributor", "ingester", "querier", "query-frontend", "query-scheduler", "index-gateway", "compactor"],
  "instance": ["localhost"], // overrides the `instance` label which is otherwise derived from the hostname and VU
});

const conf = new loki.Config(BASE_URL, 10000, 1.0, {}, labels);

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Signed-off-by: Christian Haudum christian.haudum@gmail.com

[mstoykov]

Is format a special kind of label that is send to loki either way and it know what to do with it?

[chaudum]

Yes, it is. The label defines the format of the log line.
Because of this, we can make sure, that if we filter on that label when querying, the log lines can be parsed.

[CLAassistant]

All committers have signed the CLA.

[vlad-diachenko]

LGTM. only one nit

[vlad-diachenko]

not sure that we need panic here. can we use a construction like this:

common.Throw(rt, fmt.Errorf("%s is not a valid log format", logFmt))

[chaudum]

Changed in #45