feat: Add MySQL instance address attribute

At GitHub we run MySQL in multi-node clusters and are often interested in
knowing the hostname of a node that is executing a query or command.

In order to acheive this we added an attribute to the Trilogy instrumentation
named `db.mysql.instance.address` which is populated with the hostname of the node:

<open-telemetry/opentelemetry-ruby-contrib#487>

This change adds the attribute to the database model as a MySQL specific connection attribute.

docs/database/mysql.md

@@ -0,0 +1,24 @@
+<!--- Hugo front matter used to generate the website version of this page:
+linkTitle: MSSQL
+--->
+
+# Semantic Conventions for MySQL
+
+**Status**: [Experimental][DocumentStatus]
+
+The Semantic Conventions for the [MySQL](https://www.mysql.com/) extend and override the [Database Semantic Conventions](database-spans.md)
+that describe common database operations attributes in addition to the Semantic Conventions
+described on this page.
+
+`db.system` MUST be set to `"mysql"`.
+
+## Connection-level attributes
+
+<!-- semconv db.mysql(tag=connection-level-tech-specific) -->
+| Attribute  | Type | Description  | Examples  | Requirement Level |
+|---|---|---|---|---|
+| [`db.jdbc.driver_classname`](database-spans.md) | string | The fully-qualified class name of the [Java Database Connectivity (JDBC)](https://docs.oracle.com/javase/8/docs/technotes/guides/jdbc/) driver used to connect. | `org.postgresql.Driver`; `com.microsoft.sqlserver.jdbc.SQLServerDriver` | Recommended |
+| `db.mysql.instance.address` | string | The hostname of the MySQL server instance bound to the current connection. This is useful in cases where the client is connecting to a MySQL server via a proxy or in a clustered environment. This client may obtain this value using queries like `select @@hostname`. | `mysql-e26b99z.example.com` | Recommended: If different from the `server.address` |
+<!-- endsemconv -->
+
+[DocumentStatus]: https://github.com/open-telemetry/opentelemetry-specification/tree/v1.22.0/specification/document-status.md

model/trace/database.yaml

@@ -265,6 +265,24 @@ groups:
           connecting to. This name is used to determine the port of a named instance.
         examples: 'MSSQLSERVER'
 
+  - id: db.mysql
+    prefix: db.mysql
+    type: span
+    extends: db
+    brief: >
+      Connection-level attributes for MySQL Server
+    attributes:
+      - id: instance.address
+        tag: connection-level-tech-specific
+        type: string
+        brief: >
+          The hostname of the MySQL server instance bound to the current connection.
+          This is useful in cases where the client is connecting to a MySQL server via a proxy or in a clustered environment.
+          This client may obtain this value using queries like `select @@hostname`.
+        requirement_level:
+          recommended: If different from the `server.address`
+        examples: 'mysql-e26b99z.example.com'
+
   - id: db.cassandra
     prefix: db.cassandra
     type: span
@@ -611,3 +629,4 @@ groups:
       - include: 'db.mongodb'
       - include: 'db.sql'
       - include: 'db.cosmosdb'
+      - include: 'db.mysql'