feat[next]: add atexit handler to dump the performance metrics at exit

[egparedes]

Adds an automatic (atexit-based) metrics dump facility to GT4Py so collected performance metrics can be written to disk on interpreter shutdown, controlled via a new config flag/env var (GT4PY_DUMP_METRICS_AT_EXIT).

[Copilot]

Pull request overview

Adds an automatic (atexit-based) metrics dump facility to GT4Py Next so collected performance metrics can be written to JSON on interpreter shutdown, controlled via a new config flag/env var.

Changes:

• Introduces GT4PY_DUMP_METRICS_AT_EXIT / config.DUMP_METRICS_AT_EXIT with optional auto-generated output filename.
• Registers an atexit handler in gt4py.next.instrumentation.metrics to dump metrics to JSON at exit.
• Adds a unit test covering the new _dump_metrics_at_exit() behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
src/gt4py/next/config.py	Adds configuration/env-var handling to enable dumping metrics at exit and generate a default filename.
src/gt4py/next/instrumentation/metrics.py	Implements and registers the atexit dump handler that writes metrics JSON to disk.
tests/next_tests/unit_tests/instrumentation_tests/test_metrics.py	Adds coverage for the new exit-dump handler under enabled/disabled modes.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[iomaganaris]

LGTM 👍