http.Request.ContentLength can be negative, which affects
the gateway_s3_bytes_received value in Prometheus output.
The commit only increases the value of the total received bytes
in gateway mode when r.ContentLength is greater than zero.
This PR adds support below metrics
- Cache Hit Count
- Cache Miss Count
- Data served from Cache (in Bytes)
- Bytes received from AWS S3
- Bytes sent to AWS S3
- Number of requests sent to AWS S3
Fixes#8549
