Demo for nginx access log and metrics, with GreptimeDB unified metric and event storage

[sunng87]

This demo introduces a reproducible nginx monitoring example, it contains these pieces:

• GreptimeDB, for storage
• Grafana, with GreptimeDB plugin, for visualization
• Vector, collector logs
• Nginx, what we are monitoring
• Nginx Prometheus Exporter, generates metrics for Nginx

[sunng87]

I'm working on the vector configuration and pipeline right now.

[sunng87]

@tisonkun I think we are good for an initial merge of this nginx demo. Some more features may be added but this version is already a good start.

[tisonkun]

Merged first.