fix: expose express middleware, add system-test

[ofrobots]

The express middleware was previously not publicly exposed; do so.
Also add a system-test to validate the behaviour.

[codecov]

Codecov Report

Merging #278 into master will increase coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
+ Coverage   94.18%   94.31%   +0.13%     
==========================================
  Files           4        4              
  Lines          86       88       +2     
  Branches       15       15              
==========================================
+ Hits           81       83       +2     
  Misses          2        2              
  Partials        3        3

Impacted Files	Coverage Δ
src/common.ts	97.95% <ø> (ø)	⬆️
src/index.ts	84.61% <100%> (+2.79%)	⬆️

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[ofrobots]

@JustinBeckwith the docs job failure seems like a linkinator bug.

[JustinBeckwith]

Yeah, so taking a look - "bug" is a strong word :) This url, as of now, is a 404:
https://raw.githubusercontent.com/googleapis/nodejs-logging-winston/master/doc/images/request-bundling.png

Not sure what the right way to handle this is! We could:
a.) Ignore the error and hammer this in, knowing that it will exist after the PR lands
b.) Create a commit that adds the image first, then this one
c.) Try to do some bespoke thing specific to github

Any thoughts on how y'all think this should go?

[ofrobots]

Hammer is least effort, let's go with that.

[JustinBeckwith]

SGTM

[ofrobots]

The post install check was finding a legitimate failure when the module is used with winston@2. I had to water down some of the declared types to make things work with both winston@2 and winston@3.

[ofrobots]

Post install is continuing to find legitimate problems.

node_modules/@google-cloud/logging-winston/build/src/middleware/express.d.ts:27:21 - error TS2694: Namespace 'winston' has no exported member 'Logger'.

27     logger: winston.Logger;

I thought I had addressed this by removing the manual type definition for the return type for middleware function. It turns out that the compiler will generate an explicit type anyway.

I don't see a way to emit proper types and support both winston@2 and winston@3 with the same module.

The only option I see is to drop support for winston@2 in a semver major.

[DominicKramer]

@ofrobots Let me know whenever you want another review of this. Thanks.

[ofrobots]

Had to drop require-inject and replace it with proxyquire because of this issue: https://gist.github.com/ofrobots/06270fe3f65ea93a4bda1a13348d8400

[ofrobots]

@JustinBeckwith @bcoe I need an admin to land this. The docs failure is a known limitation of the docs test.