[Azure Monitor] Check if resp.Interval for armmonitor list api can be nil

[MichaelKatsoulis]

In line

beats/x-pack/metricbeat/module/azure/monitor_service.go

Line 322 in 67e847c

interval = *resp.Interval

we are dereferencing the resp.Interval pointer to get the interval from the api response.

We do not check if the pointer is not nil.
An error reported from a client:

"error": {
       "message": "runtime error: invalid memory address or nil pointer dereference"
   },
   "stack": "github.com/elastic/elastic-agent-libs/logp.Recover\n\tgithub.xingqwq.cn/elastic/elastic-agent-libs@v0.17.4/logp/global.go:101\nruntime.gopanic\n\truntime/panic.go:770\nruntime.panicmem\n\truntime/panic.go:261\nruntime.sigpanic\n\truntime/signal_unix.go:881\ngithub.xingqwq.cn/elastic/beats/v7/x-pack/metricbeat/module/azure.(*MonitorService).GetMetricValues\n\tgithub.xingqwq.cn/elastic/beats/v7/x-pack/metricbeat/module/azure/monitor_service.go:322\ngithub.xingqwq.cn/elastic/beats/v7/x-pack/metricbeat/module/azure.(*Client).GetMetricValues\n\tgithub.xingqwq.cn/elastic/beats/v7/x-pack/metricbeat/module/azure/client.go:240\ngithub.xingqwq.cn/elastic/beats/v7/x-pack/metricbeat/module/azure.(*MetricSet).Fetch\n\tgithub.xingqwq.cn/elastic/beats/v7/x-pack/metricbeat/module/azure/azure.go:136\ngithub.xingqwq.cn/elastic/beats/v7/metricbeat/mb/module.(*metricSetWrapper).fetch\n\tgithub.xingqwq.cn/elastic/beats/v7/metricbeat/mb/module/wrapper.go:279\ngithub.xingqwq.cn/elastic/beats/v7/metricbeat/mb/module.(*metricSetWrapper).startPeriodicFetching\n\tgithub.xingqwq.cn/elastic/beats/v7/metricbeat/mb/module/wrapper.go:261\ngithub.xingqwq.cn/elastic/beats/v7/metricbeat/mb/module.(*metricSetWrapper).run\n\tgithub.xingqwq.cn/elastic/beats/v7/metricbeat/mb/module/wrapper.go:235\ngithub.xingqwq.cn/elastic/beats/v7/metricbeat/mb/module.(*Wrapper).Start.func1\n\tgithub.xingqwq.cn/elastic/beats/v7/metricbeat/mb/module/wrapper.go:175",
   "ecs.version": "1.6.0"
}

[elasticmachine]

Pinging @elastic/obs-ds-hosted-services (Team:obs-ds-hosted-services)

[MichaelKatsoulis]

Can be handled together with #43885