ci: use docker/metadata-action to gather tags/labels

[v1v]

What does this pull request do?

Use https://github.com/docker/metadata-action/tree/8e5442c4ef9f78752691e2d8f8d19755c6f78e81/?tab=readme-ov-file#typeref to get the tags/annotations/labels

Use sha commit for the github action related to docker.

Test

See https://github.com/elastic/apm-agent-python/actions/runs/8879068667

tags: docker.elastic.co/observability/apm-agent-python:test-latest
    labels: org.opencontainers.image.created=2024-04-29T13:04:44.8[6](https://github.com/elastic/apm-agent-python/actions/runs/8879068667/job/24375994029#step:6:6)6Z
  org.opencontainers.image.description=Official Python agent for Elastic APM
  org.opencontainers.image.licenses=BSD-3-Clause
  org.opencontainers.image.revision=[7](https://github.com/elastic/apm-agent-python/actions/runs/8879068667/job/24375994029#step:6:7)ea253bd5942e0e0782449edba9ee78f01a8c64a
  org.opencontainers.image.source=https://github.com/elastic/apm-agent-python
  org.opencontainers.image.title=apm-agent-python
  org.opencontainers.image.url=https://github.com/elastic/apm-agent-python
  org.opencontainers.image.version=test-latest
    build-args: AGENT_DIR=./build/dist/package/python

Related issues

https://github.com/elastic/apm-agent-nodejs/pull/3998

[v1v]

Are we publishing to docker at each merge to main?

That's correct, it was added in https://github.com/elastic/apm-agent-python/pull/1998

The tag is test-latest in that particular case