客户端库提供了四种核心度量类型
服务器没有使用类型信息，将所有数据压扁成无类型的时间序列


A counter is a cumulative metric that represents a single monotonically increasing counter 
whose value can only increase or be reset to zero on restart. 
计数器是一个累积度量，表示一个单调递增的计数器，其值只能递增或重启时重置为零。

can use a counter to represent the number of requests served, tasks completed, or errors.

Do not use a counter to expose a value that can decrease. 
For example, do not use a counter for the number of currently running processes; instead use a gauge.

https://github.com/prometheus/client_java


Gauge
A gauge is a metric that represents a single numerical value that can arbitrarily go up and down.
Gauges are typically used for measured values like temperatures or current memory usage, 
but also "counts" that can go up and down, like the number of concurrent requests.


计量器 


Histogram
A histogram samples observations (usually things like request durations or response sizes) 
and counts them in configurable buckets. It also provides a sum of all observed values.

分布    按桶计数
响应大小   请求处理时间
响应时间分布   50%  70% %80 90% 95% 99% 

A histogram with a base metric name of BASENAME exposes multiple time series 

BASENAME_bucket
BASENAME_sum
BASENAME_count


Use the histogram_quantile() function to calculate quantiles from histograms or even aggregations of histograms. 
A histogram is also suitable to calculate an Apdex score. 
When operating on buckets, remember that the histogram is cumulative. 

https://en.wikipedia.org/wiki/Apdex

Apdex  Application Performance Index



分位数

histograms and summaries
https://prometheus.io/docs/practices/histograms/

Summary
Similar to a histogram, a summary samples observations 
(usually things like request durations and response sizes).
While it also provides a total count of observations and a sum of all observed values, 
it calculates configurable quantiles over a sliding time window.
 

类似于柱状图，是对观测结果的汇总样本(通常是请求持续时间和响应大小)。
它还提供了观测总数和所有观测值的和，它通过一个滑动时间窗口计算可配置分位数。