Fix access log parsing for custom log format

[nkashiv]

Proposed changes

This PR aims to fix the bug where if a user has an access log format with a variable that is not present in the logVarMap https://github.com/nkashiv/agent/blob/main/src/core/metrics/sources/nginx_access_log.go#L33 then the access log parsing fails and no access log metric is collected.

For example, the following log format will cause an error because it contains $upstream_addr which is not present in the map. A user can also have a custom variable defined which will break the parsing.

log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for" '
                      '"$upstream_addr" "$upstream_response_time" "$upstream_status" '
                      '$http_x_amplify_id';

Added a unit test in nginx_access_log_test.go to demonstrate this fix.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• 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
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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[achawla2012]

LGTM