Generalize the writing of labeled streams

[prjemian]

When the SPEC callback is used, any ophyd object labeled as "motor" will be written to a separate stream at the start or a run and/or a separate stream at the end of a run.

bluesky_training/bluesky/instrument/callbacks/spec_data_file_writer.py

Lines 34 to 43 in 46a4dfe

	try:
	# feature new in apstools 1.6.14
	from apstools.plans import label_stream_wrapper
	def motor_start_preprocessor(plan):
	return label_stream_wrapper(plan, "motor", when="start")
	RE.preprocessors.append(motor_start_preprocessor)
	except Exception:
	logger.warning("Could load support to log motors positions.")

While SPEC data file writing needs the label_start_motor stream (so as to write the values of all motors into each scan's heading lines), the idea is not unique to SPEC files.

Move this code out of the SPEC support and generalize such that configuration is directed by a list of labels in the (iconfig.yml) file:

LABELED_STREAMS:
  start:
    - ad_metadata
    - motor

@qzhang234, @sureshnaps Thanks for the suggestion! (aps-8id-dys/bluesky@d201249)

[qzhang234]

@prjemian I'm a bit confused about how to use this with the Run Engine. Should I create a plan just like the one shown above, but replace motor_start_preprocessor with bp.count([lambda2m])), and then the plan would execute the count command and create the Nexus file?

Also, could you add Suresh and Eric to this issue as well?

[prjemian]

Comment is not related directly to this issue. The code shown above (in this GitHub issue) is for configuration of the bluesky instrument. I'll respond to the question you asked in the Teams conversation.

[prjemian]

Actually, responded in the new issue just created.