docs(samples): add get table labels

[pmakani]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

Merging #540 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #540      +/-   ##
============================================
+ Coverage     81.43%   81.49%   +0.06%     
  Complexity     1226     1226              
============================================
  Files            77       77              
  Lines          6210     6220      +10     
  Branches        681      691      +10     
============================================
+ Hits           5057     5069      +12     
- Misses          790      792       +2     
+ Partials        363      359       -4     

Impacted Files	Coverage Δ	Complexity Δ
.../google/cloud/bigquery/spi/v2/HttpBigQueryRpc.java	5.20% <0.00%> (-0.04%)	2.00% <0.00%> (ø%)
...src/main/java/com/google/cloud/bigquery/Field.java	83.49% <0.00%> (+0.16%)	27.00% <0.00%> (ø%)
...n/java/com/google/cloud/bigquery/BigQueryImpl.java	83.59% <0.00%> (+0.17%)	74.00% <0.00%> (ø%)
...google/cloud/bigquery/StandardTableDefinition.java	93.33% <0.00%> (+0.18%)	16.00% <0.00%> (ø%)
...m/google/cloud/bigquery/QueryJobConfiguration.java	90.08% <0.00%> (+0.27%)	66.00% <0.00%> (ø%)
...a/com/google/cloud/bigquery/InsertAllResponse.java	89.18% <0.00%> (+0.30%)	15.00% <0.00%> (ø%)
...om/google/cloud/bigquery/LoadJobConfiguration.java	93.20% <0.00%> (+0.43%)	56.00% <0.00%> (ø%)
...ogle/cloud/bigquery/WriteChannelConfiguration.java	86.81% <0.00%> (+0.57%)	46.00% <0.00%> (ø%)

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