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

content/en/metrics/custom_metrics/_index.md

@@ -1,5 +1,6 @@
 ---
 title: Custom Metrics
+description: "Learn what custom metrics are, how they're identified by name and tags, and how they're billed in Datadog."
 aliases:
   - /guides/metrics/
   - /metrictypes/

content/en/metrics/custom_metrics/agent_metrics_submission.md

@@ -1,5 +1,6 @@
 ---
 title: "Metric Submission: Custom Agent Check"
+description: "Submit custom metrics from a custom Agent check using count, gauge, histogram, and rate functions."
 aliases:
   - /developers/metrics/agent_metrics_submission/
   - /metrics/agent_metrics_submission

content/en/metrics/custom_metrics/historical_metrics.md

@@ -1,5 +1,6 @@
 ---
 title: Historical Metrics Ingestion
+description: "Ingest custom metric values with timestamps older than one hour, up to your metric retention period."
 further_reading:
 - link: "https://www.datadoghq.com/blog/historical-metrics/"
   tag: "Blog"

content/en/metrics/custom_metrics/powershell_metrics_submission.md

@@ -1,5 +1,6 @@
 ---
 title: "Metric submission: PowerShell"
+description: "Submit custom metrics to Datadog from PowerShell using the API or the Datadog Agent."
 aliases:
   - /developers/faq/powershell-api-examples
   - /developers/faq/submitting-metrics-via-powershell

content/en/metrics/custom_metrics/type_modifiers.md

@@ -1,5 +1,6 @@
 ---
 title: Metric Type Modifiers
+description: "Use the as_count and as_rate in-app modifiers to switch metrics between count and rate representations."
 aliases:
  - /developers/metrics/metric_type_modifiers
  - /graphing/faq/as_count_validation