Add long_field_max_length configuration option (#493)

Author: trentm

specs/agents/README.md

@@ -61,6 +61,7 @@ You can find details about each of these in the [APM Data Model](https://www.ela
 - [Agent Configuration](configuration.md)
 - [Agent logging](logging.md)
 - [Data sanitization](sanitization.md)
+- [Field limits](field-limits.md)
 
 # Processes
 

specs/agents/field-limits.md

@@ -0,0 +1,31 @@
+## Field limits
+
+The maximum length of metadata, transaction, span, et al fields are determined
+by the [APM Server Events Intake API schema](https://www.elastic.co/guide/en/apm/server/current/events-api.html).
+Fields that are names or identifiers of some resource of typically limited to
+1024 unicode characters.
+
+### `long_field_max_length` configuration
+
+Some APM event fields are not limited in the APM server intake API schema.
+Agents SHOULD limit the maximum length of the following fields by truncating.
+
+- `transaction.context.request.body`, `error.context.request.body`
+- `transaction.context.message.body`, `span.context.message.body`, `error.context.message.body`
+- `span.context.db.statement`
+- `error.exception.message`
+- `error.log.message`
+
+Agents MAY support the `long_field_max_length` configuration option to allow
+the user to configure this maximum length. This option defines a maximum number
+of unicode characters for each field.
+
+|                |           |
+|----------------|-----------|
+| Type           | `Integer` |
+| Default        | `10000` |
+| Dynamic        | `false` |
+| Central config | `false` |
+
+Ultimately the maximum length of any field is limited by the [`max_event_size`](https://www.elastic.co/guide/en/apm/server/current/configuration-process.html#max_event_size)
+configured for the receiving APM server.