Add range --count-only and --keys-only mutual exclusivity check

[nwnt]

Adding the mutual exclusivity check for Range on the server side.

Addressing the comment #21791 (comment).

[k8s-ci-robot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.20%. Comparing base (dde11c0) to head (d58b67e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
api/v3rpc/rpctypes/error.go	90.47% <ø> (ø)
server/etcdserver/api/v3rpc/key.go	77.51% <100.00%> (-0.93%)	⬇️

... and 31 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #21802      +/-   ##
==========================================
+ Coverage   70.08%   70.20%   +0.12%     
==========================================
  Files         426      426              
  Lines       35245    35247       +2     
==========================================
+ Hits        24701    24745      +44     
+ Misses       9157     9113      -44     
- Partials     1387     1389       +2     

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@nwnt: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-etcd-e2e-amd64	d58b67e	link	true	/test pull-etcd-e2e-amd64

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[serathius]

What happens currently? Do we have a test for that?

[nwnt]

From what I can see:

• on the client side, the validation is on etcdctl get command, but the same check is not performed by the client package for the Get operation.
• on the server side, the current behavior is if --count-only takes the priority where storeTxnCommon.rangeKeys returns early.

There are no tests that validate the latter behavior. The --keys-only tests I added previously avoided this case because I ran into the validation error returned by etcdctl (used by the e2e tests), but at that time I incorrectly assumed the client (used by the integration tests) also did it.

I'm thinking maybe what we should do instead is to adjust the behavior of etcdctl to stop validating this case and document it that --count-only takes precedence and we can adjust the test cases accordingly.

[serathius]

Even if client validates and server doesn't, we still need to avoid breaking server behavior. We need a server only test using clientv3 instead of etcdctl. That's why we use common package to write a test that can be executed on both paths client and command line. Please take a look how to best do that.