Change advanced metrics socket default location

[Dean-Coakley]

Proposed changes

• Change default location of advanced metrics socket:/tmp/advanced-metrics.sock -> /var/run/nginx-agent/advanced-metrics.sock
• Restrict socket permissions 0774 -> 0660

Validated that agent installed via package created socket that has a group of nginx-agent, with rw permissions:

$ ls -l /var/run/nginx-agent/
total 0
srw-rw---- 1 root nginx-agent 0 Sep  7 15:21 advanced-metrics.sock
srw-rw---- 1 root nginx-agent 0 Sep  7 15:21 nginx.sock

Checklist

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• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md)