ポイントは3つ

[Key]はユニークに
Key1つにcommandは1つだけ
-metric-key-prex はグラフのプレフィックスになるので分かりやすい名前でOK

[plugin.metrics.accesslog_default]
command = "mackerel-plugin-accesslog -metric-key-prefix default /var/log/nginx/default.access.log"

[plugin.metrics.accesslog_localhost]
command = "mackerel-plugin-accesslog -metric-key-prefix localhost /var/log/nginx/localhost.access.log"

[plugin.metrics.accesslog_exmaple_com]
command = "mackerel-plugin-accesslog -metric-key-prefix example.com /var/log/nginx/example.com.access.log"

[plugin.metrics.accesslog_api_exmaple_com]
command = "mackerel-plugin-accesslog -metric-key-prefix api.example.com /var/log/nginx/api.example.com.access.log"

[plugin.metrics.accesslog_img_exmaple_com]
command = "mackerel-plugin-accesslog -metric-key-prefix api.example.com /var/log/nginx/img.example.com.access.log"

お手軽ですね

mackerel-agent-plugins/mackerel-plugin-accesslog at master · mackerelio/mackerel-agent-plugins
mackerel-plugin-accesslog徹底解説 - Mackerel ブログ #mackerelio
mo