[DOCS-14030] Document Metric Name Pricing for Custom Metrics

[buraizu]

What does this PR do? What is the motivation?

Fixes DOCS-14030

Documents the new Metric Name pricing model for Custom Metrics. The model is mutually exclusive with the existing Timeseries (cardinality) pricing model. This PR adds:

• A new sibling billing page at content/en/account_management/billing/metric_name_pricing.md covering the model's SKUs, pricing structure, ingestion behavior, HMI billing, committed adoption, Usage Attribution, and custom/standard metric classification.
• Three diagrams (SVG sources + PNG renders) in static/images/account_management/billing/metric_name_pricing/:
  • Ingestion billing flow showing the relationship between ingested and indexed volumes
  • HMI variant showing the 1:1 ratio
  • Marginal pricing tier visualization
• Light updates to five adjacent pages, each cross-linking to the new page and scoping their existing content to the cardinality pricing model.

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

Draft PR submitted as work-in-progress for early review. Several scope items remain pending before merge readiness.

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/docs14030/metric-name-pricing/account_management/billing/metric_name_pricing

Modified Files

• https://docs-staging.datadoghq.com/docs14030/metric-name-pricing/account_management/billing/custom_metrics
• https://docs-staging.datadoghq.com/docs14030/metric-name-pricing/account_management/billing/pricing
• https://docs-staging.datadoghq.com/docs14030/metric-name-pricing/metrics/custom_metrics/
• https://docs-staging.datadoghq.com/docs14030/metric-name-pricing/metrics/custom_metrics/historical_metrics
• https://docs-staging.datadoghq.com/docs14030/metric-name-pricing/metrics/metrics-without-limits