Neko/metrics copy edits

[neko-dd]

What does this PR do? What is the motivation?

Copy edits for Metrics documentation

Merge instructions

• [ ] Please merge after reviewing

Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/neko/metrics-copy-edits/metrics/
• https://docs-staging.datadoghq.com/neko/metrics-copy-edits/metrics/custom_metrics
• https://docs-staging.datadoghq.com/neko/metrics-copy-edits/metrics/custom_metrics/agent_metrics_submission
• https://docs-staging.datadoghq.com/neko/metrics-copy-edits/metrics/custom_metrics/historical_metrics
• https://docs-staging.datadoghq.com/neko/metrics-copy-edits/metrics/custom_metrics/type_modifiers