Disallow passing types as tags

[dstrain115]

• This has caused confusion when passing a tag type with PhysicalZTag as a tag when you mean the instance PhysicalZTag().

Fixes: #3897

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.86%. Comparing base (988ecbc) to head (a39ccb9).
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6947   +/-   ##
=======================================
  Coverage   97.86%   97.86%           
=======================================
  Files        1084     1084           
  Lines       94408    94408           
=======================================
+ Hits        92393    92397    +4     
+ Misses       2015     2011    -4     

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[pavoljuhas]

LGTM, thanks! Please consider updating a few comments before merging -

diff --git a/cirq-google/cirq_google/api/v2/program.proto b/cirq-google/cirq_google/api/v2/program.proto
index cbc66cc4..c0b4dc32 100644
--- a/cirq-google/cirq_google/api/v2/program.proto
+++ b/cirq-google/cirq_google/api/v2/program.proto
@@ -146 +146 @@ message ZPowGate {
-  // cirq.ZPowGate(...).with_tags(cirq.google.PhysicalZTag)
+  // cirq.ZPowGate(...).with_tags(cirq.google.PhysicalZTag())
diff --git a/cirq-google/cirq_google/transformers/target_gatesets/google_cz_gateset.py b/cirq-google/cirq_google/transformers/target_gatesets/google_cz_gateset.py
index dacefe8c..fe59e26b 100644
--- a/cirq-google/cirq_google/transformers/target_gatesets/google_cz_gateset.py
+++ b/cirq-google/cirq_google/transformers/target_gatesets/google_cz_gateset.py
@@ -40 +40 @@ class GoogleCZTargetGateset(cirq.CZTargetGateset):
-             `cirq_google.PhysicalZTag`) from single-qubit Pauli operations. Defaults to False.
+             `cirq_google.PhysicalZTag()`) from single-qubit Pauli operations. Defaults to False.