fix: remove circular references

[stephenplusplus]

Fixes: googleapis/nodejs-logging#381

Please see: googleapis/google-cloud-node#1987 (comment)

Also see the matching PR that logging-bunyan received, that logging-winston did not:
googleapis/google-cloud-node#2094

[ofrobots]

LGTM.

For posterity: The rationale provided on googleapis/google-cloud-node#2094 is no longer true because now we default options.inspectMetadata to be false which means that the premise that circularity will be removed by util.inspect no longer holds.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@90a1a31). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master    #264   +/-   ##
========================================
  Coverage          ?   92.5%           
========================================
  Files             ?       3           
  Lines             ?      80           
  Branches          ?      24           
========================================
  Hits              ?      74           
  Misses            ?       2           
  Partials          ?       4

Impacted Files	Coverage Δ
src/common.ts	98.24% <100%> (ø)

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