Fix unhashable dict tag crashing triggerer metrics (#66353)

[jakubmatyszewski]

• closes: Triggerer keeps crashing after DeadlineAlert is triggered with OTeL enabled #66353

The triggerer crashes with a TypeError: unhashable type: 'dict' when OpenTelemetry metrics are enabled.

Root cause

The crash occurs because Callback.get_metric_info() in airflow/models/callback.py can return a tags dict containing a nested dict as a value (the "kwargs" key). OpenTelemetry's _view_instrument_match.py later calls frozenset(attributes.items()) to build an aggregation key, which fails because dict values are not hashable.

Fix

Serialize the remaining kwargs to a string, or drop them if empty.

How to reproduce

1. Start Airflow with OTeL enabled (the OTel endpoint doesn't need to be reachable - the crash happens before any export).
2. Spawn task that will trigger DeadlineAlert.

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.