SofaRpcMetricsCollector可能产生npe

[Just-CJ]

Describe the bug

SofaRpcMetricsCollector收集指标数据时依赖了的部分上下文参数可能获取不到，可能导致npe

Expected behavior

rpc/common-default.log中无异常

Actual behavior

rpc/common-default.log中会出现npe错误

java.lang.NullPointerException: null
	at com.alipay.sofa.rpc.metrics.prometheus.SofaRpcMetricsCollector$PrometheusSubscriber.onEvent(SofaRpcMetricsCollector.java:249)
	at com.alipay.sofa.rpc.metrics.prometheus.SofaRpcMetricsCollector$PrometheusSubscriber.onEvent(SofaRpcMetricsCollector.java:229)
	at com.alipay.sofa.rpc.event.EventBus.handleEvent(EventBus.java:153)
	at com.alipay.sofa.rpc.event.EventBus.post(EventBus.java:123)
	at com.alipay.sofa.rpc.client.ClientProxyInvoker.invoke(ClientProxyInvoker.java:91)
	at com.alipay.sofa.rpc.api.GenericService_proxy_0.$genericInvoke(GenericService_proxy_0.java)
	at com.alipay.sofa.circus.common.GenericServiceTest.lambda$init$1(GenericServiceTest.java:66)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:750)

Steps to reproduce

Minimal yet complete reproducer code (or GitHub URL to code)

Environment

• SOFARPC version: 5.13.1
• JVM version (e.g. java -version):
• OS version (e.g. uname -a):
• Maven version:
• IDE version: