Add a metric to measure "number of open files"

[madeesha]

Description:
APIM gateway shows a large number of open files when running a load test. Checked the number of open files using, lsof command and sar -v. The results are mentioned below.

output of 'lsof | wc -l' before start the server : 879

output of 'lsof | wc -l' just after start the server : 53590

output of 'lsof | wc -l' when running the load test : 4674652

output of 'lsof -p pid | wc -l' when running the load test : 3969

sar -v output

06:26:01 56643 1152 66043 2
06:27:01 56643 1120 66043 2
06:28:01 56643 1120 66043 2
06:29:01 56643 1120 66043 2
06:30:01 56643 1152 66043 2
06:31:01 56643 1152 66043 2

Affected Product Version: APIM 2.6.0

OS, DB, other environment details and versions:
OS - Ubuntu
DB - PostregeSQL