Support sending metrics via Prometheus remote_write protocol in SDK

[Looka149]

What would you like to be added:
Support for sending metrics via the Prometheus remote_write protocol in the JuiceFS SDK.
This includes adding two new configuration options:

• juicefs.push-remote-write: The remote_write endpoint URL
• juicefs.push-remote-write-auth: Authentication information for the remote_write endpoint

Why is this needed:
Currently, the SDK supports sending metrics only to Pushgateway and Graphite.
By adding support for the Prometheus remote_write protocol, JuiceFS can integrate directly with systems that natively support remote_write, such as VictoriaMetrics' vmagent and Prometheus.