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/database-spans.md

@@ -65,6 +65,7 @@ Some database systems may allow a connection to switch to a different `db.user`,
 | Attribute  | Type | Description  | Examples  | Requirement Level |
 |---|---|---|---|---|
 | `db.connection_string` | string | The connection string used to connect to the database. It is recommended to remove embedded credentials. | `Server=(localdb)\v11.0;Integrated Security=true;` | Recommended |
+| `db.instance.id` | string | An identifier (address, unique name, or any other identifier) of the database instance that is executing queries or mutations on the current connection. This is useful in cases where the database is running in a clustered environment and the instrumentation is able to record the node executing the query. The client may obtain this value in databases like MySQL using queries like `select @@hostname`. | `mysql-e26b99z.example.com` | Recommended: If different from the `server.address` |
 | `db.system` | string | An identifier for the database management system (DBMS) product being used. See below for a list of well-known identifiers. | `other_sql` | Required |
 | `db.user` | string | Username for accessing the database. | `readonly_user`; `reporting_user` | Recommended |
 | [`network.peer.address`](../attributes-registry/network.md) | string | Peer address of the network connection - IP address or Unix domain socket name. | `10.1.2.80`; `/tmp/my.sock` | Recommended |

model/trace/database.yaml

@@ -246,6 +246,16 @@ groups:
         tag: connection-level
       - ref: network.type
         tag: connection-level
+      - id: instance.id
+        tag: connection-level
+        type: string
+        brief: >
+          An identifier (address, unique name, or any other identifier) of the database instance that is executing queries or mutations on the current connection.
+          This is useful in cases where the database is running in a clustered environment and the instrumentation is able to record the node executing the query.
+          The client may obtain this value in databases like MySQL using queries like `select @@hostname`.
+        requirement_level:
+          recommended: If different from the `server.address`
+        examples: 'mysql-e26b99z.example.com'
 
   - id: db.mssql
     prefix: db.mssql