ref(internal): Remove sentry._internal from conventions - #185

Description

As discussed, sentry._internal should be reserved for the EAP data model, when converting from a span/log/.. to EAP, not for actual attributes.

In a future change, we'll introduce a way to hide attributes.

PR Checklist

• I have run yarn test and verified that the tests pass.
• I have run yarn generate && yarn format to generate and format code and docs.

If an attribute was added:

• The attribute is in a namespace (e.g. nextjs.function_id, not function_id)
• I've used the sentry._internal namespace if the attribute should not be visibile to end-users
• I have used the correct value for pii (i.e. maybe or true. Use false only for values that should never be scrubbed such as IDs)

If an attribute was deprecated:

• I've followed the policies described in CONTRIBUTING.md