[V1][Metrics] Support vllm:cache_config_info

[markmc]

Part of #10582

prometheus_client has support for Info metrics which are equivalent to a Gauge whose value is permanently set to 1, but exposes interesting key/value pair information via labels. This is used for information about an instance that does not change - so it only needs to be observed at startup - and allows comparing across instances in Prometheus.

We use this concept for the vllm:cache_config_info metric:

vllm:cache_config_info{block_size="16",cache_dtype="auto",calculate_kv_scales="False",cpu_offload_gb="0",enable_prefix_caching="False",gpu_memory_utilization="0.9",...} 1.0

However, prometheus_client has never supported Info metrics in multiprocessing mode - for unclear reasons. We simply use a Gauge metric set to 1 and multiprocess_mode="mostrecent" instead.

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[robertgshaw2-redhat]

This is one we should consider adding to the deprecation list.