Custom labels

[SrDayne]

Is there a way to pass environmental variables as custom labels of metrics?

How metric looks now:
php_fpm_up{instance="example", socket_path="example"} 1

How I expect it to see:
php_fpm_up{instance="example", socket_path="example", custom_label_1="example_1",custom_label_2="example_2"} 1,
where custom labels passed as env vars, like: custom_label_1=$env_var_1, custom_label_2=$env_var_2

For reference, see:
https://github.com/nginxinc/nginx-prometheus-exporter, command line argument -prometheus.const-labels value