Close connection to scrape target after retrieving data (#39)

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.10.0 as builder
+FROM golang:1.13.11 as builder
 RUN mkdir -p /go/src/github.com/cloudposse/prometheus-to-cloudwatch
 WORKDIR /go/src/github.com/cloudposse/prometheus-to-cloudwatch
 COPY . .

prometheus_to_cloudwatch.go

@@ -514,6 +514,7 @@ func fetchMetricFamilies(
 	}
 	client := &http.Client{Transport: transport}
 	decodeContent(client, url, ch)
+	client.CloseIdleConnections()
 }
 
 func decodeContent(client *http.Client, url string, ch chan<- *dto.MetricFamily) {