docs(samples): add quickstart code for ExportAssets

[peter-zheng-g]

Fixes #<issue_number_goes_here> (it's a good idea to open an issue first for discussion)

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[peter-zheng-g]

While the code can pass test with ava, it always timeout when testing with mocha.

[peter-zheng-g]

Thanks for your review.

1. There're lint errors. Do you know which package I need to install to run lint locally?
2. Can you please help enable the API for test project(s)? Error message:
   Cloud Asset API has not been used in project 1046198160504 before or it is disabled. Enable it by visiting https://console.developers.google.com/apis/api/cloudasset.googleapis.com/overview?project=1046198160504

[JustinBeckwith]

Sure! To lint, you can just run npm run fix from the cwd. I am guessing you need to add something like this to the .eslintrc.yml in the samples/system-test directory:

---
env:
  mocha: true

See this as an example:
https://github.com/googleapis/nodejs-pubsub/blob/master/samples/system-test/.eslintrc.yml

[JustinBeckwith]

Also, I enabled the API 👍

[peter-zheng-g]

I still can't fun lint locally, it complains about missing script:
$ npm run fix
npm ERR! missing script: fix

npm ERR! A complete log of this run can be found in:
npm ERR! /usr/local/google/home/peterzh/.npm/_logs/2018-11-19T23_04_38_650Z-debug.log

[peter-zheng-g]

OK, added fix at top level package.json (locally) and now I can run "npm run fix". Thanks.

[JustinBeckwith]

Goodness, apologies. For now you could run npx eslint --fix '**/*.js'

[peter-zheng-g]

Now both test and lint pass on my local machine. Thanks!

[codecov-io]

Codecov Report

Merging #50 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #50   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines           4      4           
=====================================
  Hits            4      4

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