Add a doc for virtual database. (#8798)

CHANGES.md

@@ -180,6 +180,7 @@ NOTICE, this sharding concept is NOT just for splitting data into different data
 * Remove `All` source relative document and examples.
 * Update Booster UI's dependency licenses.
 * Add profiling doc, and remove service mesh intro doc(not necessary).
+* Add a doc for virtual database.
 
 All issues and pull requests are [here](https://github.com/apache/skywalking/milestone/112?closed=1)
 

docs/en/setup/service-agent/virtual-database.md

@@ -0,0 +1,15 @@
+# Virtual Database
+
+Virtual databases represents the database nodes detected by [server agents' plugins](server-agents.md). The performance
+metrics of the databases are also from Database client side perspective.
+
+For example, JDBC plugins(MySQL, PostgreSQL, Mariadb, MSSQL) in the Java agent could detect the latency of SQL
+performance, as well as SQL statements. As a result, in this dashboard, SkyWalking would show database traffic, latency,
+success rate and sampled slow SQLs powered by backend analysis capabilities.
+
+The Database access span should have
+- It is an **Exit** span
+- **Span's layer == DATABASE**
+- Tag key = `db.statement`, value = SQL statement
+- Tag key = `db.type`, value = the type of Database
+- Span's peer is the network address(IP or domain) of Database server.

docs/menu.yml

@@ -165,6 +165,10 @@ catalog:
                 path: "/en/setup/backend/backend-vm-monitoring"
           - name: "Browser Monitoring"
             path: "/en/setup/service-agent/browser-agent"
+          - name: "Database Monitoring"
+            catalog:
+              - name: "Virtual Database"
+                path: "en/setup/service-agent/virtual-database"
           - name: "Self Observability"
             catalog:
               - name: "OAP Telemetry"