brpc的brpc_metrics的输出格式与prometheus的标准不符，尤其MultiDimension<bvar::LatencyRecorder>方式

[xinhua5]

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

使用MultiDimensionbvar::LatencyRecorder 自定义了监控埋点，输出之后看到的结果是这样的：

# TYPE server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="80"} gauge
server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="80"} 4076
# HELP server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="90"}
# TYPE server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="90"} gauge
server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="90"} 5185
# HELP server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="99"}
# TYPE server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="99"} gauge
server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="99"} 5185
# HELP server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="999"}
# TYPE server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="999"} gauge
server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="999"} 5185
# HELP server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="9999"}
# TYPE server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="9999"} gauge
server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="9999"} 5185

Describe the solution you'd like (描述你期望的解决方法)

prometheus期望的格式是这样的：

# HELP server_latency Latency of zbase server operations
# TYPE server_latency gauge
server_latency{db_id="112",shard_id="1",method="fetch",stage="next",quantile="99"} 2816
server_latency{db_id="112",shard_id="1",method="fetch",stage="next",quantile="999"} 2816
server_latency{db_id="112",shard_id="1",method="fetch",stage="next",quantile="9999"} 2816
server_latency{db_id="112",shard_id="2",method="fetch",stage="next"} 2729
server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="80"} 4076
server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="90"} 5185
server_latency{db_id="112",shard_id="2",method="fetch",stage="next",quantile="99"} 5185

Describe alternatives you've considered (描述你想到的折衷方案)
brpc是否可以把brpc_metrics的输出格式转化为 metrics + lables 的模式

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

[Huixxi]

@serverglen cc，也可以尝试提交个PR进行优化。

[snehavats1404]

can i work on this? @chenBright

[chenBright]

can i work on this? @chenBright

OK. By the way, I'm also fixing this issue.