Don't start heartbeat until _writer is set (#4062)

[erik-megarad]

What do these changes do?

As seen in #4062 . self._reset_heartbeat() is being called during _pre_start, which is before the writer is set. The error in the issue will occur if the writer isn't set before the first heartbeat is sent.

This change waits until the writer is set, during _post_start, to start sending heartbeats.

Are there changes in behavior for the user?

None

Related issue number

#4062

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
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  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
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    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[asvetlov]

Thanks!