[EMSUSD-3043] Setting stage metadata value results in a Ill-formed SdfPath warning message

**Describe the bug**
Setting stage metadata value results in a Ill-formed SdfPath warning message

**Steps to reproduce**
Steps to reproduce the behavior:

Execute this in maya's python editor:

```
import maya.cmds as cmds
import mayaUsd.lib as mayaUsdLib

cmds.createNode('mayaUsdProxyShape', skipSelect=True, name="abc")
stage = mayaUsdLib.GetPrim("abc").GetStage()
stage.SetMetadata('metersPerUnit', 1)
```
Result :

`# Warning: Ill-formed SdfPath <>: :1:1: parse error matching struct pxrInternal_v0_25_5__pxrReserved__::Sdf_PathParser::Path`

However, my stage metadata actually reflects the correct value.
Why do I get this warning message ?

**Specs (if applicable):**
- OS & version: Windows 11
- Maya version Maya 2026.3
- Maya USD 0.34.5
- Pixar USD 25.05