Report non-fatal collection errors in the exporter metric. (#1439)

As per prometheus/client_golang#543, pass the Registry for exporter
metrics when setting up the /metrics HTTP handler.

With this, the `promhttp_metric_handler_errors_total` metric will
increment on (possibly non-fatal) collection-time errors, such as
duplicate metrics from text files.

Signed-off-by: Matthias Rampke <mr@soundcloud.com>

node_exporter.go

@@ -116,6 +116,7 @@ func (h *handler) innerHandler(filters ...string) (http.Handler, error) {
 			ErrorLog:            log.NewErrorLogger(),
 			ErrorHandling:       promhttp.ContinueOnError,
 			MaxRequestsInFlight: h.maxRequests,
+			Registry:            h.exporterMetricsRegistry,
 		},
 	)
 	if h.includeExporterMetrics {