add DialCustom for overridden runtime behavior

[demmer]

Description

Add a new DialCustom hook to the vtgateconn.

We have a case where we want to create a grpcvtgateconn with a dynamic dialer to include runtime options, specifically to override the load balancer using the following:

	withBalancer := grpc.WithDefaultServiceConfig(...)
	dialer := grpcvtgateconn.Dial(withBalancer)

Rather than registering this dialer in the global set, we want to instead just invoke the dialer function inline using this newly-added hook:

	conn, err := vtgateconn.DialCustom(ctx, dialer, target)

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.49%. Comparing base (3d7f63e) to head (335ff58).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/vtgateconn/vtgateconn.go	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18733      +/-   ##
==========================================
- Coverage   67.50%   67.49%   -0.01%     
==========================================
  Files        1605     1605              
  Lines      263756   263764       +8     
==========================================
- Hits       178046   178032      -14     
- Misses      85710    85732      +22     

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[arthurschreiber]

Minor nitpick, but it would be nice if these tests used require:

Suggested change

	_, err := DialProtocol(context.Background(), protocol, "")
	if err != nil || dialer != "default" {
	t.Fatalf("default dialerFunc should have been called, got %s, err: %v", dialer, err)
	}
	_, err = DialCustom(context.Background(), customDialerFunc, protocol)
	if err != nil || dialer != "custom" {
	t.Fatalf("custom dialerFunc should have been called, got %s, err: %v", dialer, err)
	}
	_, err = DialCustom(context.Background(), customDialerFunc2, protocol)
	if err != nil || dialer != "custom2" {
	t.Fatalf("custom2 dialerFunc should have been called, got %s, err: %v", dialer, err)
	}
	_, err := DialProtocol(context.Background(), protocol, "")
	require.NoError(t, err)
	require.Equal(t, dialer, "default", "default dialerFunc should have been called")
	_, err = DialCustom(context.Background(), customDialerFunc, protocol)
	require.NoError(t, err)
	require.Equal(t, dialer, "custom", "custom dialerFunc should have been called")
	_, err = DialCustom(context.Background(), customDialerFunc2, protocol)
	require.NoError(t, err)
	require.Equal(t, dialer, "custom2", "custom2 dialerFunc should have been called")

[demmer]

Ah much better thanks. Updated.