mbvar设定了多label，某些label对应的value是不断增长的，比如label:table,version，如果服务一直运行，最终会达到bvar_max_dump_multi_dimension_metric_number，这时新的bvar就不能写入，这种场景下，是不是改成lru，淘汰旧的比较好。

**Is your feature request related to a problem? (你需要的功能是否与某个问题有关?)**
mbvar达到上限

**Describe the solution you'd like (描述你期望的解决方法)**
达到bvar_max_dump_multi_dimension_metric_number后淘汰旧bvar

**Describe alternatives you've considered (描述你想到的折衷方案)**

**Additional context/screenshots (更多上下文/截图)**

Start by locating the mbvar implementation and the bvar_max_dump_multi_dimension_metric_number limit. Determine how multi-label bvars are stored and whether the limit currently rejects new entries. Done means defining and implementing an eviction policy so older bvars are removed at the limit and new bvars can be recorded.

Written by the indexing model from the issue text.