counter/histogram with enabledExemplars=true, the `labels()` function does not bind the input parameters properly

[jdmarshall]

				const counterInstance = new Counter({
					name: 'counter_exemplar_label_test',
					help: 'help',
					labelNames: ['method', 'code'],
					enableExemplars: true,
				});

				const labels = counterInstance.labels({ method: 'get', code: '200' });

				labels.inc({}); // 1
				labels.inc({
					exemplarLabels: { traceId: 'trace_id_test', spanId: 'span_id_test' },
				});

There's a performance issue here since labels() should be able to validate the label values and then not repeat them. Likely what needs to happen is the two paths need to be unwound or converted into a set of file scoped functions that result in the correct output, sort of like how observe() does. That would both address the issue here and provide an incentive to use the labels() function to reduce overhead per call.

This code is exactly duplicated into Histogram and has the exact same bug.

[jdmarshall]

I believe the right way to solve this is to modify labels() to skip over revalidation of inputs for those that cannot change rather than using bind the way it is at the moment.

Those changes should also result in the internals of observe() and inc being homogenized so that side-by-side comparison of the files shows the way that histogram exemplars and counter exemplars are identical or different. So that any subsequent additions to this code are easier to reason about.

For these reasons I'm putting myself as the assignee.

[jdmarshall]

I'm not planning to implement #407, but I might be able to make it less difficult for someone else to do so.