Clarification on span name cardinality

Author: AlexanderWert

specs/agents/tracing-instrumentation-db.md

@@ -18,7 +18,7 @@ The following fields are relevant for database and datastore spans. Where possib
 
 | Field | Description | Mandatory |
 |-------|-------------|:---------:|
-|`name`| The name of the exit database span. For SQL operations we perform a limited parsing of the statement, and extract the operation name and outer-most table involved. Other databases and storages may have different strategies for the span name (see specific databases and stores in the sections below).| :white_check_mark: |
+|`name`| The name of the exit database span. **The span name must have a low cardinality as it is used as a dimension for derived metrics!** Therefore, for SQL operations we perform a limited parsing of the statement, and extract the operation name and outer-most table involved. Other databases and storages may have different strategies for the span name (see specific databases and stores in the sections below).| :white_check_mark: |
 |`type`|For database spans, the type should be `db`.| :white_check_mark:|
 |`subtype`|For database spans, the subtype should be the database vendor name. See details below for specific databases.| :x: |
 |`action`|The database action, e.g. `query`|