fix(multiprocess): avoid double-building child metric names (#1035)

[hazel-shen]

Description

Fixes #1035

Child metrics created via labels() could rebuild their full names when using metric subclasses with default namespace/subsystem/unit, especially in multiprocess mode, producing double-prefixed names.

Problem

With a subclass like:

classCustomCounter(Counter):
def__init__(self, name, documentation, labelnames=(),
namespace='mydefaultnamespace',
subsystem='mydefaultsubsystem',
unit='defaultunit',
**kwargs):
super().__init__(name, documentation, labelnames=labelnames,
namespace=namespace, subsystem=subsystem,
unit=unit, **kwargs)

and an instance:

c=CustomCounter(
name='m',
documentation='help',
labelnames=('status', 'method'),
namespace='mynamespace',
subsystem='mysubsystem',
unit='seconds',
)

multiprocess mode could export child metrics with double-prefixed names like:

mydefaultnamespace_mydefaultsubsystem_mynamespace_mysubsystem_m_seconds_total

instead of the correct:

mynamespace_mysubsystem_m_seconds_total

This happened because the already-built full name was passed to the child metric constructor, which then applied namespace/subsystem/unit transformations a second time.

Solution

• In MetricWrapperBase.__init__, store original constructor arguments:

  self._original_name=nameself._namespace=namespaceself._subsystem=subsystemself._unit=unit

• In MetricWrapperBase.labels(), use stored values when creating child metrics:

  original_name=getattr(self, '_original_name', self._name)
  namespace=getattr(self, '_namespace', '')
  subsystem=getattr(self, '_subsystem', '')
  unit=getattr(self, '_unit', '')
  self._metrics[labelvalues] =self.__class__(
  original_name,
  namespace=namespace,
  subsystem=subsystem,
  unit=unit,
  ...
  )

This ensures the full name is built exactly once, making single-process and multiprocess exports consistent.

Testing

• All existing tests pass
• Verified correct name construction in multiprocess mode
• Verified subclass compatibility (child metrics preserve parent context)
• Backward compatible via getattr() fallbacks

Breaking Changes

None (internal-only change to MetricWrapperBase)

[csmarchbanks]

Thanks!