✨ 🔧 [Telemetry] Add metrics to measure health, latency, request rate to model providers

[roma-glushko]

Measure these metrics for all model providers configured:

• the number of successful requests (counter)
• the number of failed requests (counter)
• the response latency (non-streaming lang chat requests)
• request rate to each provider
• the first chunk latency (streaming lang chat requests)

[gernest]

request rate to each provider

rate is a derived metric, it is computed by the time series store. Having total_requests counter is enough, you can compute rate of requests from it.