[BEAM-7735] Label ParDos with their name

[happygiraffe]

The diagram is much easier to read if individual ParDos are identifiable.

This applies to the Go SDK's dot runner. It was not previously possible to obtain the name.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---

Pre-Commit Tests Status (on master branch)

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

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[happygiraffe]

R: @lostluck

[lostluck]

Please update the PR description that this is for the Go SDK dot runner, and that previously it wasn't possible
Please prefix the PR title with [BEAM-7735] to refer to https://issues.apache.org/jira/browse/BEAM-7735 which is a subtask I've created for other Dot runner improvements.

[happygiraffe]

Please update the PR description that this is for the Go SDK dot runner, and that previously it wasn't possible

Done.

Please prefix the PR title with [BEAM-7735] to refer to https://issues.apache.org/jira/browse/BEAM-7735 which is a subtask I've created for other Dot runner improvements.

Done.