net/http/pprof: add AttachHandlers method to net/http/pprof to support non-default muxes

[DerekTBrown]

Separate the net/http/pprof package init function into a separate AttachHandlers method to better support non-default muxes.

Fixes #71213.

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Patch Set 1:

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Message from Ian Lance Taylor:

Patch Set 1: Hold+1

(2 comments)

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