prometheus: stop requiring localstack-core at runtime

[carole-lavillonniere]

Motivation

The prometheus extension is about to be published to PyPI for the first time, as #178 switched its README to a PyPI install now that git URL installs are gone (localstack/localstack-pro#9309).

It requires localstack-core>=4.2.0,<5.0.0 at runtime, but LocalStack now ships calendar versions (2026.9.1), so installing it pulls localstack-core 4.14.0 from PyPI into the extensions environment, next to the real one.

Ticket: COSY-1084

Changes

• pyproject.toml: move localstack-core from the runtime dependencies to the dev extra. LocalStack provides it at runtime, as for the other extensions.
• Makefile: make dist ran python setup.py, but there is no setup.py; build with python -m build.

Tests

• Built the wheel and installed it with LocalStack's installer in localstack-pro:latest: before, pip installed localstack-core-4.14.0; now only the extension and prometheus-client.
• Not tested: loading the extension in a running LocalStack. It imports LocalStack Pro modules, which are only importable once LocalStack has started with a license.