Fix NR's count metric by using float rather than int

[taonic]

What was changed

1. Submit float value to NewRelic's count metrics.

https://docs.newrelic.com/docs/data-apis/understand-data/metric-data/metric-data-type/

2. Remove the unwanted server close callback.

Why?

Prometheus's rate function calculates the per-second average rate of increase of the time series in the range vector, which is returned as a decimal number in json. For example, with low RPS, e.g. 8 requests in 1 minute, the rate query returns 8/60 ~= 0.133. Therefore, we should submit the number to NewRelic as float rather than int.

Checklist

1. Closes N/A

2. How was this tested: Manual

3. Any docs updates needed? No