fix gauge registration with Micrometer #847
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Gauges in Micrometer by default hold the state object weakly. Since we used the gauge value supplier as the state object, extracted the gauge value from the state object, and the supplier isn't used anywhere else, it was easily possible for the supplier to be garbage collected, leading to `NaN` reported gauge values. With this commit, we no longer rely on the gauge state object. Instead, the function that extracts the gauge value from the state object refers to the value supplier directly. We still need to pass a non-`null` state object to Micrometer, otherwise the gauge would never have a value. And for the gauge to keep having a value, that state object must be strongly reachable from somewhere. Since the value extraction function is held strongly by Micrometer, and the function holds a strong reference to the supplier, we use the supplier as the state object, even if we don't use it for anything else.
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