[DOCS-9330] Add intro to generate metrics processor (#25948)

* add intro

* Update layouts/shortcodes/observability_pipelines/processors/generate_metrics.en.md

Co-authored-by: Sandra (neko) <[email protected]>

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Co-authored-by: Sandra (neko) <[email protected]>

layouts/shortcodes/observability_pipelines/processors/generate_metrics.en.md

@@ -1,4 +1,4 @@
-This processor generates either a count metric of logs that match a query or a distribution metric of a numeric value contained in the logs, such as a request duration.
+Many types of logs are meant to be used for telemetry to track trends, such as KPIs, over long periods of time. Generating metrics from your logs is a cost-effective way to summarize log data from high-volume logs, such as CDN logs, VPC flow logs, firewall logs, and networks logs. Use the generate metrics processor to generate either a count metric of logs that match a query or a distribution metric of a numeric value contained in the logs, such as a request duration.
 
 **Note**: The metrics generated are [custom metrics][10031] and billed accordingly. See [Custom Metrics Billing][10032] for more information.