enhancement(observability): Add _utilization_mean buffer metrics

[bruceg]

Summary

The source and transform buffer utilization metrics currently measure either the
instantaneous level or a histogram, neither of which is a reliable single-valued
reflection of the actual status. For this purpose, we introduce a mean metric
that uses the EWMA calculations to produce a simple moving average that will
better track the actual value.

Vector configuration

[sources.metrics]
type = "internal_metrics"

[transforms.filter]
type = "filter"
inputs = ["metrics"]
condition.type = "is_metric"

[sinks.console]
type = "console"
inputs = ["filter"]
encoding.codec = "text"

How did you test this PR?

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Notes

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[datadog-vectordotdev]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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[maycmlee]

Just a couple of small suggestions

[thomasqueirozb]

Nice!

[thomasqueirozb]

Suggested change

	let mut result = f64::NAN;
	self.average
	.fetch_update(Ordering::Relaxed, Ordering::Relaxed, |current| {
	let average = if current.is_nan() {
	point
	} else {
	point.mul_add(self.alpha, current * (1.0 - self.alpha))
	};
	result = average;
	average
	});
	result
	self.average
	.fetch_update(Ordering::Relaxed, Ordering::Relaxed, |current| {
	if current.is_nan() {
	point
	} else {
	point.mul_add(self.alpha, current * (1.0 - self.alpha))
	}
	})

I think this would work the same as fetch_update returns an f64

[bruceg]

The issue is that fetch_update returns the old value (hence the mnemonics of fetch-then-update), but we want to return the updated average which is only accessible in the closure or by adding another fetch which adds to the expense.