chore: better document that qps stats aren't real-time

Addresses #4.

TL;DR: NS1 qps stats are time delayed, not real time. Updates README and
the help text of the metric itself to make it clearer.

relevant docs:

https://ns1.com/api?docId=2361

README.md

@@ -53,6 +53,8 @@ _Note_: While packages are built for several systems, there are currently no pla
 
 The primary purpose of the exporter is to expose NS1 DNS queries-per-second stats from the NS1 API.
 
+> :warning: _NOTE_: The queries-per-second statistics available from the NS1 API are not real-time, but are time delayed. This means that the metric values that are exposed to Prometheus are not real-time. This exporter makes no attempt to adjust metric timestamp to try and align the corresponding timestamp with the qps values.
+
 An example Prometheus configuration file demonstrating how to scrape metrics can be found in [docs/examples/prometheus_ns1_metrics.yml](./docs/examples/prometheus_ns1_metrics.yml)
 
 ### Metrics

pkg/metrics/metrics.go

@@ -36,7 +36,7 @@ var (
 	)
 	MetricQPSDesc = prometheus.NewDesc(
 		prometheus.BuildFQName(metricNamespace, "stats", "queries_per_second"),
-		"DNS queries per second for the labeled NS1 resource.",
+		"DNS queries per second for the labeled NS1 resource. Note that NS1 QPS metrics are time delayed, not real-time.",
 		[]string{"zone_name", "record_name", "record_type"}, nil,
 	)