Add metrics traits

[jwright6323]

This is a draft of the metrics mix-in traits.

[hammer-robot]

Step Static type checker failed with details below:

Running mypy --package hammer_vlsi
Running mypy --package hammer_utils
Running mypy ../hammer-shell/get-config
Running mypy ../hammer-vlsi/cli_driver_test.py ../hammer-vlsi/constraints_test.py ../hammer-vlsi/lef_utils_test.py ../hammer-vlsi/tech_test.py ../hammer-vlsi/test.py ../hammer-vlsi/units_test.py ../hammer-vlsi/utils_test.py ../hammer-vlsi/verilog_utils_test.py
Running mypy ../hammer_config_test/test.py
Running mypy ../hammer-vlsi/synthesis/mocksynth/__init__.py
Running mypy ../hammer-vlsi/synthesis/vivado/__init__.py
Running mypy ../hammer-vlsi/synthesis/nop.py
Running mypy ../hammer-vlsi/par/nop.py
Running mypy ../hammer-vlsi/par/vivado/__init__.py
Running mypy ../hammer-vlsi/drc/mockdrc.py ../hammer-vlsi/drc/nop.py
Running mypy ../hammer-vlsi/lvs/mocklvs.py ../hammer-vlsi/lvs/nop.py
/buildbot/run_unittests/build/src/hammer-vlsi/hammer_vlsi/hammer_metrics.py:72: error: invalid syntax

[jwright6323]

• Also solve Port parameter of clock constraints is not fully vendor-independent #280 here

[colinschmidt]

I think it might be better to break the fix for #280 out from a new metrics API. Just from a CR perspective I can imagine it getting confusing.

[edwardcwang]

I agree with @colinschmidt here - in general for CI/CR it's nicer to have lots of small PRs than big PRs. (Sometimes it can't be helped though)