[DOCS-14454] Add description front matter to metrics guide pages

[buraizu]

What does this PR do? What is the motivation?

Fixes DOCS-14454

Adds the description field to YAML front matter on 10 pages under content/en/metrics/guide/ that lacked it. Per the Datadog docs front matter guidance, description is required on every page.

Companion PR to #36884 (top-level metrics pages). Three more PRs to follow for metrics/custom_metrics/, metrics/faq/, and metrics/open_telemetry/.

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/docs14454/metrics-guide-descriptions/metrics/guide/agent-filtering-for-custom-metrics
• https://docs-staging.datadoghq.com/docs14454/metrics-guide-descriptions/metrics/guide/calculating-the-system-mem-used-metric
• https://docs-staging.datadoghq.com/docs14454/metrics-guide-descriptions/metrics/guide/custom_metrics_governance
• https://docs-staging.datadoghq.com/docs14454/metrics-guide-descriptions/metrics/guide/different-aggregators-look-same
• https://docs-staging.datadoghq.com/docs14454/metrics-guide-descriptions/metrics/guide/dynamic_quotas
• https://docs-staging.datadoghq.com/docs14454/metrics-guide-descriptions/metrics/guide/interpolation-the-fill-modifier-explained
• https://docs-staging.datadoghq.com/docs14454/metrics-guide-descriptions/metrics/guide/micrometer
• https://docs-staging.datadoghq.com/docs14454/metrics-guide-descriptions/metrics/guide/rate-limit
• https://docs-staging.datadoghq.com/docs14454/metrics-guide-descriptions/metrics/guide/what-is-the-granularity-of-my-graphs-am-i-seeing-raw-data-or-aggregates-on-my-graph
• https://docs-staging.datadoghq.com/docs14454/metrics-guide-descriptions/metrics/guide/why-does-zooming-out-a-timeframe-also-smooth-out-my-graphs

[janine-c]

Suggested change

	description: "Understand how interpolation and the fill modifier align metric series so you can aggregate across sources."
	description: "Understand how interpolation and the fill modifier help you aggregate across sources by aligning metric series."

I wasn't familiar with interpolation and the fill modifier beforehand, so I had some trouble parsing out where the unfamiliar noun phrase ended. I'm not sure if this slight rewording helps; feel free to ignore if you prefer your current version!