chore: release 6.4.0

[release-please]

🤖 I have created a release *beep* *boop*

6.4.0 (2021-04-29)

Features

• adds getValue to ResultSet (#1073) (7792c90)

Bug Fixes

• allow using case-insensitive user-agent key (#1110) (f4f9e43)
• check for timeout in connection after last statement finished (#1086) (aec0b54), closes #1077
• check for timeout in connection after last statement finished (#1086) (51d753c), closes #1077
• do not keep references to invalidated clients (#1093) (b4595a6), closes #1089
• prevent potential NullPointerException in Struct with Array field that contains null elements (#1107) (c414abb), closes #1106

Dependencies

• update dependency com.google.cloud:google-cloud-monitoring to v2.2.1 (#1104) (37ca990)
• update dependency com.google.cloud:google-cloud-trace to v1.3.3 (#1103) (b4327c0)

Documentation

• fix javadoc for Date type (#1102) (ce095f7)
• use default timeout for restore operation (#1109) (3f3c13e), closes #1019

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[codecov]

Codecov Report

Merging #1099 (f1608f3) into master (f4f9e43) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1099   +/-   ##
=========================================
  Coverage     84.98%   84.99%           
- Complexity     2727     2730    +3     
=========================================
  Files           156      156           
  Lines         14416    14416           
  Branches       1381     1381           
=========================================
+ Hits          12252    12253    +1     
+ Misses         1596     1594    -2     
- Partials        568      569    +1     

Impacted Files	Coverage Δ	Complexity Δ
...m/google/cloud/spanner/connection/SpannerPool.java	87.16% <0.00%> (-0.54%)	33.00% <0.00%> (ø%)
...a/com/google/cloud/spanner/SessionPoolOptions.java	69.53% <0.00%> (ø)	18.00% <0.00%> (+1.00%)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.01% <0.00%> (+0.19%)	75.00% <0.00%> (+2.00%)

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[yoshi-automation]

🤖 Release is at https://github.com/googleapis/java-spanner/releases/tag/v6.4.0 🌻

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