spanner-jdbc: Add integration tests for JDBC DatabaseMetadata

[olavloite]

Adds integration tests for JDBC DatabaseMetadata.

[codecov]

Codecov Report

Merging #6035 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #6035      +/-   ##
============================================
- Coverage      47.1%   47.02%   -0.08%     
+ Complexity    27180    25715    -1465     
============================================
  Files          2523     2523              
  Lines        274440   269305    -5135     
  Branches      30557    31187     +630     
============================================
- Hits         129271   126641    -2630     
+ Misses       134770   133989     -781     
+ Partials      10399     8675    -1724

Impacted Files	Coverage Δ	Complexity Δ
...re/src/main/java/com/google/cloud/BaseService.java	55.55% <0%> (-6.95%)	2% <0%> (ø)
...m/google/cloud/vision/v1/ImageAnnotatorClient.java	58.69% <0%> (-3.31%)	15% <0%> (-4%)
...e/cloud/vision/v1p4beta1/ImageAnnotatorClient.java	58.69% <0%> (-3.31%)	15% <0%> (-4%)
...le/cloud/compute/deprecated/DeprecationStatus.java	88.63% <0%> (-3.13%)	20% <0%> (ø)
...e/cloud/vision/v1p3beta1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
...e/cloud/vision/v1p2beta1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
.../java/com/google/cloud/speech/v1/SpeechClient.java	48.57% <0%> (-2.78%)	10% <0%> (-2%)
...om/google/cloud/speech/v1p1beta1/SpeechClient.java	48.57% <0%> (-2.78%)	10% <0%> (-2%)
...d/webrisk/v1beta1/WebRiskServiceV1Beta1Client.java	63.41% <0%> (-2.5%)	12% <0%> (-3%)
...ogle/cloud/texttospeech/v1/TextToSpeechClient.java	55.88% <0%> (-2.46%)	9% <0%> (-2%)
... and 666 more

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[kolea2]

LGTM after rebase

[olavloite]

Rebased.