Switch Metric-Resource relationship to back_populates

[Callum027]

Replace usage of the deprecated backref option [1] and implicit relationship population, with the new back_populates [2] option and explicit relationship definitions on both the Metric and Resource classes.

This eliminates "AttributeError: type object 'Metric' has no attribute 'resource'" exceptions that can occur when Metric.resource is referenced using sqlalchemy.orm.joinedload in queries.

This change is backwards compatible with SQLAlchemy 1.4.

1: https://docs.sqlalchemy.org/en/20/orm/backref.html
2: https://docs.sqlalchemy.org/en/20/orm/basic_relationships.html

[Callum027]

Originally encountered while troubleshooting a separate issue with python-gnocchiclient.

gnocchixyz/python-gnocchiclient#142 (comment)

[Callum027]

Test failing because of an unrelated dependency toolchain issue with the version of pkg_resources (and jaraco.text) being used.

Traceback (most recent call last):
  File "/github/workspace/.tox/py39-postgresql-file-upgrade-from-4.4/bin/gnocchi", line 5, in <module>
    from gnocchiclient.shell import main
  File "/github/workspace/.tox/py39-postgresql-file-upgrade-from-4.4/lib/python3.9/site-packages/gnocchiclient/shell.py", line 40, in <module>
    from gnocchiclient.version import __version__
  File "/github/workspace/.tox/py39-postgresql-file-upgrade-from-4.4/lib/python3.9/site-packages/gnocchiclient/version.py", line 15, in <module>
    import pkg_resources
  File "/github/workspace/.tox/py39-postgresql-file-upgrade-from-4.4/lib/python3.9/site-packages/pkg_resources/__init__.py", line 96, in <module>
    from jaraco.text import (
ImportError: cannot import name 'yield_lines' from 'jaraco.text' (/usr/lib/python3/dist-packages/jaraco/text/__init__.py)
py39-postgresql-file-upgrade-from-4.4: exit 1 (45.47 seconds) /github/workspace> run-upgrade-tests.sh postgresql-file pid=120
  py39-postgresql-file-upgrade-from-4.4: FAIL code 1 (52.61=setup[7.14]+cmd[45.47] seconds)
  evaluation failed :( (52.67 seconds)

Pull request has been modified.