OpenTelemetry: update metric + attribute names

[sean-breen]

Proposed changes

Renames some NGINX OSS + Nginx Plus metrics to be more verbose.

Occurrences of the abbreviation conn for connections have been changed to connections in most places. This includes in attribute names.

Old metric	New metric
nginx.http.conn	nginx.http.connections

Old attribute	New attribute
nginx.conn.outcome	nginx.connections.outcome

The only ones not affected are when the abbreviation matches a directive name in nginx configuration, such as limit_conn.

Also update the Grafana dashboards in our mock environment to use the new metric names, and leverage the instance.type resource attribute to filter based on Nginx instance type.

Checklist

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• 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