Register MapCoder, some comments/cleanup.

[robertwb]

R: @kileys
CC: @katsiapis

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ValidatesRunner compliance status (on master branch)

Lang	ULR	Dataflow	Flink	Samza	Spark	Twister2
Go	---					---
Java
Python	---					---
XLang						---

Examples testing status on various runners

Lang	ULR	Dataflow	Flink	Samza	Spark	Twister2
Go	---	---	---	---	---	---	---
Java	---		---	---	---	---	---
Python	---	---	---	---	---	---	---
XLang	---	---	---	---	---	---	---

Post-Commit SDK/Transform Integration Tests Status (on master branch)

Go	Java	Python

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website	Whitespace	Typescript
Non-portable
Portable	---			---	---	---

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GitHub Actions Tests Status (on master branch)

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[lukecwik]

LGTM but some of the tests seemed to have strict name matching:
NameError: name '_CoGroupByKey__CoGBKImpl' is not defined

[lukecwik]

R: @lukecwik

[robertwb]

Thanks. That turned out to be a typo with the double underscore. I'll make sure tests are green before merging.

[robertwb]

I'm getting failures in apache_beam.io.gcp.experimental.spannerio_test.SpannerReadTest.test_read_with_transaction. Commented in JIRA.

[robertwb]

Run Python PreCommit

[codecov]

Codecov Report

Merging #15471 (90ec784) into master (823ab5a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #15471   +/-   ##
=======================================
  Coverage   83.74%   83.75%           
=======================================
  Files         443      443           
  Lines       60077    60089   +12     
=======================================
+ Hits        50312    50326   +14     
+ Misses       9765     9763    -2     

Impacted Files	Coverage Δ
sdks/python/apache_beam/pipeline.py	91.66% <ø> (ø)
sdks/python/apache_beam/coders/coders.py	88.58% <100.00%> (+0.59%)	⬆️
sdks/python/apache_beam/coders/row_coder.py	95.03% <100.00%> (ø)
sdks/python/apache_beam/coders/typecoders.py	94.52% <100.00%> (+0.07%)	⬆️
...ython/apache_beam/io/gcp/experimental/spannerio.py	88.20% <100.00%> (ø)
sdks/python/apache_beam/transforms/util.py	95.81% <100.00%> (ø)
sdks/python/apache_beam/utils/interactive_utils.py	87.80% <0.00%> (-7.32%)	⬇️
...he_beam/runners/interactive/sql/beam_sql_magics.py	54.70% <0.00%> (-0.96%)	⬇️
...eam/runners/interactive/interactive_environment.py	90.70% <0.00%> (-0.38%)	⬇️
.../python/apache_beam/portability/api/metrics_pb2.py	100.00% <0.00%> (ø)
... and 7 more

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