* plugin/metrics: set server address in context
Allow cross server block metrics to co-exist; for this we should label
each metric with the server label. Put this information in the context
and provide a helper function to get it out.
Abstracting with entirely away with difficult as the release client_go
(0.8.0) doesn't have the CurryWith functions yet. So current use is like
so:
define metric, with server label:
RcodeCount = prometheus.NewCounterVec(prometheus.CounterOpts{
Namespace: plugin.Namespace,
Subsystem: "forward",
Name: "response_rcode_count_total",
Help: "Counter of requests made per upstream.",
}, []string{"server", "rcode", "to"})
And report ith with the helper function metrics.WithServer:
RcodeCount.WithLabelValues(metrics.WithServer(ctx), rc, p.addr).Add(1)
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