[Lens] Page for Metric charts (#3645)

Fixes #2543

Here is a draft of a page for metrics charts. Would like reviewers to
advise on anything missing and inaccuracies in particular for the
settings part where there may be more nuance to add in some places.

Let's finetune what we can and once we have a few different chart pages
ready, we can release in small batches

Done:

- [x] Optimize structure for readability
- [x] Analyze competition, check discuss forums, test product and check
existing materials to decide on optimal content depth
- [x] Extract best practices from EUI's dataviz guidelines
- [x] Test current content - doesn't mean the doc is exhaustive, just
that what's in there is correctly reflecting the product, there may be
more nuance to add edge cases.
- [x] Have doc reviewed for completeness and accuracy
- [x] Have visualization best practices checked

Missing:
- ~refine examples (various use cases but also various configuration
setups)~ Done and checked
- ~visuals for examples~ Done and checked
- ~add some advanced, undocumented options that don't look so simple for
defining values~ Done and checked

Release plan:
- We'll release these as hidden for now, and turn it on once we have a
batch of pages that makes sense (metric, line, bar, area, time series
pages at least)
- We'll remove duplicated content from main Lens page when turning those
pages on

---------

Co-authored-by: Nastasha Solomon <[email protected]>
Co-authored-by: Brandon Morelli <[email protected]>

Author: 3 people

.gitignore

@@ -12,4 +12,7 @@ AGENTS.md
 .github/instructions/**.instructions.md
 CLAUDE.md
 GEMINI.md
-.cursor
+.cursor
+
+# VS code settings
+.vscode

explore-analyze/_snippets/lens-breakdown-advanced-settings.md

@@ -0,0 +1,9 @@
+:::{dropdown} Advanced settings
+Several advanced options allow you to refine the behavior of the breakdown:
+
+- **Include documents without the selected field**: Off by default.
+- **Group remaining values as "Other"**: On by default.
+- **Enable accuracy mode**: This option improves results for high-cardinality data, but increases the load on the {{es}} cluster.
+- **Include values**: Values from the breakdown dimension to always show a tile for.
+- **Exclude values**: Values from the breakdown dimension to always exclude from the displayed tiles.
+:::

explore-analyze/_snippets/lens-value-advanced-settings.md

@@ -0,0 +1,9 @@
+:::{dropdown} Advanced settings
+Depending on the data you defined, several options allow you to apply additional filtering to the data taken into account to compute the metric's value:
+    
+- **Normalize by unit**: Normalize the metric values to show per unit of time.
+- **Filter by**: Specify a query.
+- **Reduced time range**: Reduce the time range specified on the dashboard's time filter by the specified duration.
+- **Time shift**: Shift the time range by the specified duration. This is useful if the value should use a different time range than the one selected on the dashboard.
+- **Hide zero values**: Don't show values equal to zero. This option is on by default.
+:::