Use appropriate context to call NGINX Plus API

[lucacome]

https://github.com/nginxinc/nginx-plus-go-client/releases/tag/v2.0.0 now supports passing a context to the API calls, so we need to pass a context to c.nginxClient.GetStats()

nginx-prometheus-exporter/collector/nginx_plus.go

Lines 621 to 625 in 262228e

	func (c *NginxPlusCollector) Collect(ch chan<- prometheus.Metric) {
	c.mutex.Lock() // To protect metrics from concurrent collects
	defer c.mutex.Unlock()
	stats, err := c.nginxClient.GetStats()

Figure out if we want to pass a context or a timeout. We already have a timeout that users can set for scraping metrics

nginx-prometheus-exporter/exporter.go

Line 95 in 262228e

timeout = createPositiveDurationFlag(kingpin.Flag("nginx.timeout", "A timeout for scraping metrics from NGINX or NGINX Plus.").Default("5s").Envar("TIMEOUT").HintOptions("5s", "10s", "30s", "1m", "5m"))

it probably makes sense to use the same value.

[sjberman]

I think that makes sense to use as the same timeout.