cgroup max memory discovery analysis (#2746)

Author: eyalkoren

apm-agent-core/src/main/java/co/elastic/apm/agent/metrics/builtin/CGroupMetrics.java

@@ -203,6 +203,7 @@ private CgroupFiles createCgroup1Files(File memoryMountPath) throws IOException
     private File getMaxMemoryFile(File maxMemoryFile, String cgroupUnlimitedConstant) throws IOException {
         try(BufferedReader maxFileReader = new BufferedReader(new FileReader(maxMemoryFile))) {
             String memMaxLine = maxFileReader.readLine();
+            logger.debug("max cgroup memory read from {} is: {}", maxMemoryFile.getAbsolutePath(), memMaxLine);
             if (cgroupUnlimitedConstant.equalsIgnoreCase(memMaxLine)) {
                 // Make sure we don't send the max metric when cgroup is not bound to a memory limit
                 maxMemoryFile = null;