Add metadata into single sweep of cirq-google/cirq_google/api/v2/run_context.proto

[BichengYing]

The motivation of adding this metadata field (will replace DeviceParameter at least internal usage) is because of the headache during the development of cirq server.

This is mainly to address the following three issues:

• One sweep may associated with multiple registry parameters (compared with using zip, this is more compact representation)
• We may want different column name from the symbol name (or reg_parameter).
• We may or may not want to include the axes value in the returned dataset. (Internally, we use Const or Var to represent it, it is better to be explicitly in the proto instead)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.86%. Comparing base (83a8e0e) to head (9bcfa14).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6869      +/-   ##
==========================================
- Coverage   97.86%   97.86%   -0.01%     
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  Files        1084     1084              
  Lines       94290    94290              
==========================================
- Hits        92280    92279       -1     
- Misses       2010     2011       +1     

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[mhucka]

This looks reasonable to me. My only concern is about testing the changes. Pardon my ignorance w.r.t. this part of the code, but is there any way to add test cases for this change?

[BichengYing]

This looks reasonable to me. My only concern is about testing the changes. Pardon my ignorance w.r.t. this part of the code, but is there any way to add test cases for this change?

About the test, it will be added after we have functional code calling this protobuf. (The python code generated here are auto-generated).

[senecameeks]

LGTM % nits