Update gauge example in documentation

[zeelshah90]

In this doc[1] section 9.1 please change the example from

AtomicInteger n = registry.gauge("numberGauge", new AtomicInteger(0));
n.set(1);
n.set(2);

TO

AtomicInteger atomicInteger = new AtomicInteger(0);
AtomicInteger n = registry.gauge("numberGauge", atomicInteger);
n.set(1);
n.set(2);

As above example does not work due to weakreference of new AtomicIntger(0).
[1] https://micrometer.io/docs/concepts#_gauges

[izeye]

@zeelshah90 The example you mentioned seems to be valid for me if you retain any reference to n. Am I missing something?

[shakuzen]

I have updated the example code with comments that hopefully clarify things. Let me know if anyone thinks further clarification is still necessary.