oximeter log messages can intersperse with propolis-server log messages

This is the same thing as #407, but for oximeter; this call creates its own logger instead of using a child logger:
https://github.com/oxidecomputer/propolis/blob/b59b355a18691af30641ce2d12b723abb3d8b9c1/bin/propolis-server/src/lib/stats.rs#L147

which results in log message interleaving:

Aug 25 21:55:34.920 INFO listening, local_addr: 127.0.0.1:37557, component: dropshot
Aug 25 21:55:34.961 WARN Aug 25 21:55:34.961Could not connect to oximeter (retrying in Error registering as metric producer: Communication Error: error sending request for url (http://127.0.0.1:9000/metrics/producers): error trying to connect: tcp connect error: Connection refused (os error 146)s):
1 , INFOuri :received request to begin graceful shutdown , /instancelocal_addr, :method :127 .0PUT.0, .1req_id:37557:,  component62d5fd2e-f857-413a-827e-40e39666e014:,  remote_addrdropshot:
 127.0.0.1:46236, local_addr: 127.0.0.1:8000
Aug 25 21:55:35.963 INFO listening, local_addr: 127.0.0.1:60856, component: dropshot
Aug 25 21:55:35.993 Aug 25 21:55:35.993WARN  INFOCould not connect to oximeter (retrying in Error registering as metric producer: Communication Error: error sending request for url (http://127.0.0.1:9000/metrics/producers): error trying to connect: tcp connect error: Connection refused (os error 146)s):
1 , received request to begin graceful shutdownuri, :local_addr :/instance , 127method.0:.0 .1PUT:60856, , req_idcomponent::  62d5fd2e-f857-413a-827e-40e39666e014dropshot, 
remote_addr: 127.0.0.1:46236, local_addr: 127.0.0.1:8000

Start at bin/propolis-server/src/lib/stats.rs around line 147 and compare the related issue #407. Reproduce the oximeter and propolis-server logging scenario, then verify that the resulting messages no longer intersperse.