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Add labeled Prometheus metrics to distinguish where prompt tokens come
from in P/D disaggregated deployments.

In P/D disaggregation, decode instances receive KV cache from prefill instances.
Currently, decode reports inflated prompt throughput because it counts all
prompt tokens as "computed", even though most were transferred.

This PR adds labeled metrics so users can understand actual compute work vs
transferred work:

vllm:prompt_tokens_by_source_total{source="local_compute"}        # Tokens prefilled locally
vllm:prompt_tokens_by_source_total{source="external_kv_transfer"} # Tokens received via KV transfer  
vllm:prompt_tokens_by_source_total{source="local_cache_hit"}      # Tokens from local prefix cache
vllm:prompt_tokens_cached_total                                    # Total cached (local + external, -1 when all 
Signed-off-by: Zhanqiu Hu <zh338@cornell.edu>