fix(db): Fix encrypted field metrics table_name tag

[vgrozdanic]

Fix encrypted field metrics to properly send the table name in metric tags.

Problem

The encrypted field metrics were not sending the model/table name correctly. The metric tag used getattr(self, "_model_name", "unknown"), but _model_name was never being set, so all metrics showed "unknown" for the model name. Additionally, the tag was named "model" which is too generic.

Root Cause

The original code implemented __set_name__() to capture the model name, expecting Python's descriptor protocol to call it. However, Django's ModelBase metaclass removes fields from the class dict and stores them in _meta before Python can call __set_name__(). Django uses contribute_to_class() instead, which the encrypted field wasn't implementing.

Changes

• Initialize _model_name in __init__ with default value "unknown" (for standalone field usage in tests)
• Implement contribute_to_class() to capture the actual model name when field is added to a Django model
• Change metric tag from "model" to "table_name" for better clarity
• Add tests to verify metrics are sent with correct tags including table_name

Testing

Added two new tests:

• test_encrypted_char_field_metrics_on_encrypt: Verifies encryption metrics have correct table_name tag
• test_encrypted_char_field_metrics_on_decrypt: Verifies decryption metrics have correct table_name tag

All 68 encryption field tests pass.