doc: Add Metrics section to the user-guide (#18216)

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- Part of #16602

## Rationale for this change

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Now we have to search in the code comment (or even implementation) to
find the documentation of certain metrics, it would be better to open a
page in the `user-guide` for metrics.

The doc has to be manually updated, the metrics construction is
scattered in the codebase, so it's hard to make it auto-generated.

This PR only includes 2 common metrics, I plan to add more
operator-specific metrics while working on
#18116

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Author: 2010YOUY01

docs/source/index.rst

@@ -122,6 +122,7 @@ To get started, see
    user-guide/sql/index
    user-guide/configs
    user-guide/explain-usage
+   user-guide/metrics
    user-guide/faq
 
 .. _toc.library-user-guide:

docs/source/user-guide/metrics.md

@@ -0,0 +1,37 @@
+<!---
+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
+  distributed with this work for additional information
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+
+    http://www.apache.org/licenses/LICENSE-2.0
+
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+  under the License.
+-->
+
+# Metrics
+
+DataFusion operators expose runtime metrics so you can understand where time is spent and how much data flows through the pipeline. See more in [EXPLAIN ANALYZE](sql/explain.md#explain-analyze).
+
+## Common Metrics
+
+### BaselineMetrics
+
+`BaselineMetrics` are available in most physical operators to capture common measurements.
+
+| Metric          | Description                                            |
+| --------------- | ------------------------------------------------------ |
+| elapsed_compute | CPU time the operator actively spends processing work. |
+| output_rows     | Total number of rows the operator produces.            |
+
+## Operator-specific Metrics
+
+TODO