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<p style="margin: 10px 0 0 0">As discussed on the call, changing this so that the metrics are tracked more in a "per-component" style (per component ID) rather than "per-instance". The latter would be basically impossible for the component in isolation to do, due to the component/bean life cycles and order-of-operations: we absolutely don't always destroy the current component instance before we init the new component instance (e.g. reloadable services in the IdP).</p>
<p style="margin: 10px 0 0 0">On init, it seems the semantic here for registering a <tt>Gauge</tt> isn't really "getOrAdd" as in the other metrics. It has to be whether to replace the existing registered metric. That's because the gauges, due to the callback nature, often will have either an explicit or implicit reference to the actual containing component instance (e.g. gauge is a non-static inner class or an anonymous class). So when init-ing, it doesn't matter whether there is an existing metric with the given name - the init-ing instance should always replace it with its own.</p>
<p style="margin: 10px 0 0 0">And then in destroy: Unlike like what I initially thought to do - remove all the metric remove() calls - you need to (sometimes) remove the metric, in particular the gauges. Essentially, because of nested/inner class usage, to avoid leaks due to circular references and resultant garbage collection issues, you always need to remove the instance's gauge. Except you can't just blindly remove by name, because it's possible (indeed likely in the reloadable service case) that the one that is registered is the "new" component which has already registered its own new one... So, you need to de-register conditionally, only if the one that is registered is the actual instance you registered. That means you need to store the gauge at init time for later use during destroy.</p>
<p style="margin: 10px 0 0 0">To support these more complex register and remove cases, and to also have thread safety, I added some overloaded methods to <tt>MetricsSupport</tt>. When those methods are used, they synchronize on the registry instance.</p>
<p style="margin: 10px 0 0 0">The <tt>register</tt> ones take a <tt>replaceExisting</tt> boolean arg, doing the obvious thing.</p>
<p style="margin: 10px 0 0 0">The <tt>remove</tt> ones take an optional <tt>Metric</tt> instance arg, and if non-null, will only remove if the registered instance is the same as the specified instance. Note: this assumes that the registry doesn't do something like wrap the the metric instances in proxies, adapters, etc. If it does, then this isn't going to work as-is. But the actual logic is factored into a separate method <tt>isMetricInstanceRegisteredUnderName</tt>, so we can customize there if needed.</p>
<p style="margin: 10px 0 0 0">Would be good if Scott or others would check my thinking on all this, since it's getting quite a bit more complicated than it originally seemed.</p>
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