Add some metrics for kvcache

[aftersnow]

Motivation

Add more metrics for tracking kvcache related information.

Modifications

The new metrics are:

• mempool_size: the size of the memory pool
• mempool_available_siz: the available size of the memory pool
• tree_cache_size: the size of the tree cache
• tree_cache_evictable_size: the evictable size of the tree cache
• retract_count: the number of retracted requests

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• (Seems no need) Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
• For reviewers: If you haven't made any contributions to this PR and are only assisting with merging the main branch, please remove yourself as a co-author when merging the PR.
• Please feel free to join our Slack channel at https://slack.sglang.ai to discuss your PR.

[aftersnow]

pls see #7030