背景

基于springboot的微服务进行系统监控改造，在现有的erueka+actuator基础上拓展Prometheus进行更多指标的系统监控，能在服务不可用前进行告警

引入依赖

actuator的依赖+Prometheus的register的依赖

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>
<dependency>
    <groupId>io.micrometer</groupId>
    <artifactId>micrometer-registry-prometheus</artifactId>
</dependency>

配置只暴露Pormetheus的端口，metric tag的配置个人觉得没有必要

management:
  endpoints:
    web.exposure.include: prometheus

到了这里之后正常GET请求 ip:port/{servlet.context-path}/actuator/prometheus
就会得到这样的回复(只截取了一点)
Content-Type:text/plain; version=0.0.4;charset=utf-8

# HELP system_cpu_usage The "recent cpu usage" for the whole system
# TYPE system_cpu_usage gauge
system_cpu_usage 0.0
# HELP process_start_time_seconds Start time of the process since unix epoch.
# TYPE process_start_time_seconds gauge
process_start_time_seconds 1.606481846248E9

然而在实际的过程中却出现了406错误

{
    "timestamp": "",
    "status": 406,
    "error": "Not Acceptable",
    "message": "Could not find acceptable representation",
    "path": "/actuator/prometheus",
}

搜了网上的答案说因为缺失MediaType(Text/Plain)导致无法解析
因为项目中自定义了HttpMessageConverter，加入了MappingJackson2HttpMessageConverter，而其默认方法只加载了MediaType Application/Json,一度以为问题就结束了，在为MappingJackson2HttpMessageConverter添加了Text/Plain的支持后，能得到格式不正确的回复(换行被转义为\n)

"# HELP jvm_memory_used_bytes The amount of used memory\n# TYPE jvm_memory_used_bytes gauge\njvm_memory_used_bytes{area=\"heap\",id=\"PS Survivor Space\",} 0.0\njvm_memory_used_bytes{area=\"heap\",id=\"PS Old Gen\",} 3.9645672E7\njvm_memory_used_bytes{area=\"heap\",id=\"PS Eden Space\",} 1.77528552E8\njvm_memory_used_bytes{area=\"nonheap\",id=\"Metaspace\",} 6.9426912E7\njvm_memory_used_bytes{area=\"nonheap\",id=\"Code Cache\",} 1.302752E7\njvm_memory_used_bytes{area=\"nonheap\",id=\"Compressed Class Space\",} 9426568.0\n

显然这不是增加一个MediaType能解决的问题，推测应该是该类所需的MessageConvertor没有被加载，导致换行被错误的转义。
因为数据的写入类是通过写入char(32)进行换行操作的，按道理说应该是会被正确的转义，于是将重写的类注释掉后运行能得到正确的答案，这就确立的问题排查的方向，找出所需的convertor进行手动加载。项目中的MessageConvertor是基于WebMvcConfigurationSupport进行重写的
然而Spring框架的拓展更多是以Adapter为结尾的

public class WebMvcConfigurationSupport implements ApplicationContextAware, ServletContextAware {
/**
     * Adds a set of default HttpMessageConverter instances to the given list.
     * Subclasses can call this method from {@link #configureMessageConverters}.
     * @param messageConverters the list to add the default message converters to
     */
    protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
        StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
        stringHttpMessageConverter.setWriteAcceptCharset(false);  // see SPR-7316

        messageConverters.add(new ByteArrayHttpMessageConverter());
        messageConverters.add(stringHttpMessageConverter);
        messageConverters.add(new ResourceHttpMessageConverter());
        messageConverters.add(new ResourceRegionHttpMessageConverter());
        try {
            messageConverters.add(new SourceHttpMessageConverter<>());
        }
        catch (Throwable ex) {
            // Ignore when no TransformerFactory implementation is available...
        }
        messageConverters.add(new AllEncompassingFormHttpMessageConverter());

        if (romePresent) {
            messageConverters.add(new AtomFeedHttpMessageConverter());
            messageConverters.add(new RssChannelHttpMessageConverter());
        }

        if (jackson2XmlPresent) {
            Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.xml();
            if (this.applicationContext != null) {
                builder.applicationContext(this.applicationContext);
            }
            messageConverters.add(new MappingJackson2XmlHttpMessageConverter(builder.build()));
        }
        else if (jaxb2Present) {
            messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
        }

        if (jackson2Present) {
            Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.json();
            if (this.applicationContext != null) {
                builder.applicationContext(this.applicationContext);
            }
            messageConverters.add(new MappingJackson2HttpMessageConverter(builder.build()));
        }
        else if (gsonPresent) {
            messageConverters.add(new GsonHttpMessageConverter());
        }
        else if (jsonbPresent) {
            messageConverters.add(new JsonbHttpMessageConverter());
        }

        if (jackson2SmilePresent) {
            Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.smile();
            if (this.applicationContext != null) {
                builder.applicationContext(this.applicationContext);
            }
            messageConverters.add(new MappingJackson2SmileHttpMessageConverter(builder.build()));
        }
        if (jackson2CborPresent) {
            Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.cbor();
            if (this.applicationContext != null) {
                builder.applicationContext(this.applicationContext);
            }
            messageConverters.add(new MappingJackson2CborHttpMessageConverter(builder.build()));
        }
    }
}

可以在没有重写情况下，框架会写入一定数量的MessageConvertor，而因为项目中的重写导致Prometheus所需的convertor没有被加载，经过一系列的排查，最终锁定了

StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
        stringHttpMessageConverter.setWriteAcceptCharset(false);

于是在重写的方法中加上该转换器后问题得到了解决。

因此引申出来的思考

WebMvcConfigurationSupport与WebMvcConfigurer的区别
如果要在spring的基础上拓展可以implements WebMvcConfigurer，
相反 如果想要一个纯净的环境，那么可以直接覆盖WebMvcConfigurationSupport
ProMetheus的配置等下一章吧。