Add support for a legacy mode for metrics

[namannandan]

🚀 The feature

Enable backwards compatibility for metrics behavior in versions <=0.8.0 by adding support for a new metrics mode: legacy

Motivation, pitch

In versions <=0.8.0:

• ts_inference_requests_total, ts_inference_latency_microseconds and ts_queue_latency_microseconds are only available via the metrics API endpoint in prometheus format.
• All other metrics are only logged to ts_metrics.log and model_metrics.log

In versions >0.8.0:

• Based on metrics_mode configuration, all metrics are either logged to ts_metrics.log & model_metrics.log or they are all made available via the metrics API endpoint in prometheus format.

Behavior of legacy metrics mode:

• ts_inference_requests_total, ts_inference_latency_microseconds and ts_queue_latency_microseconds are only available via the metrics API endpoint in prometheus format.
• All other metrics inclusive of auto detected backend metrics are only logged to ts_metrics.log and model_metrics.log

Alternatives

No response

Additional context

No response