ARCHBOM-1490: rename "custom metric" to "custom attribute"

[robrap]

Description:

Renamed "custom metric" to "custom attribute" throughout the monitoring library. This decision can be read about in the ADR 0002-custom-monitoring-language.rst. The following have been deprecated:

• set_custom_metric (use set_custom_attribute)
• set_custom_metrics_for_course_key (use set_custom_attributes_for_course_key)
• MonitoringCustomMetricsMiddleware (use CachedCustomMonitoringMiddleware)
• CachedCustomMonitoringMiddleware.accumulate_metric (use CachedCustomMonitoringMiddleware.accumulate_attribute)
• CodeOwnerMetricMiddleware (use CodeOwnerMonitoringMiddleware)

JIRA:

ARCHBOM-1490

Testing instructions:

• Tested locally in edx-platform using branch robrap/ARCHBOM-1490-upgrade-edx-django-utils
• Sandbox deploys are busted. Otherwise, will test on https://robrap.sandbox.edx.org/ against New Relic.

Merge checklist:

• [x] All reviewers approved
• [x] CI build is green
• [x] Test against sandbox
• [X] Version bumped
• [X] Changelog record added
• [X] Documentation updated (not only docstrings)
• [ ] Commits are squashed

Post merge:

• [ ] Create a tag
• [ ] Check new version is pushed to PyPi after tag-triggered build is finished.
• [ ] Delete working branch (if not needed anymore)