Fix CPU metrics (#2413)

Author: trask

agent/azure-monitor-exporter/src/main/java/com/azure/monitor/opentelemetry/exporter/implementation/utils/CpuPerformanceCounterCalculator.java

@@ -51,6 +51,8 @@ public Double getCpuPercentage() {
       if (prevUpTime > 0L && upTime > prevUpTime) {
         long elapsedCpu = processCpuTime - prevProcessCpuTime;
         long elapsedTime = upTime - prevUpTime;
+        prevUpTime = upTime;
+        prevProcessCpuTime = processCpuTime;
         // if this looks weird, here's another way to write it:
         // (elapsedCpu / 1000000.0) / elapsedTime / 100.0
         return elapsedCpu / (elapsedTime * 10_000.0);