fix: set callable name on RetryingCaller Prometheus labels

[ltsyk]

Summary

RetryingCaller / AsyncRetryingCaller always labeled Prometheus metrics as callable="<context block>" because they reused retry_context(), which hard-codes that name.

Change

Build the retry iterator via _RetryContextIterator.from_params(..., name=guess_name(callable_)) so metrics match the decorator path.

Test plan

• Module imports after change
• Instrumentation label now uses the real callable name

Fixes #110

[hynek]

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