Add Processing algorithms to deal with metadata

[alexbruy]

Description

Add a few native algorithms to deal with layer metadata:

• Copy layer metadata to copy metadata from an input layer to a target layer
• Apply layer metadata to set layer metadata from a QMD file
• Export layer metadata to export layer metadata in a QMD file

When copying and applying metadata it is possible to set metadata as a default metadata for a layer.

Supersedes #59375.

[qgis-bot]

@alexbruy
This pull request has been tagged as requiring documentation.

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[qgis-bot]

@alexbruy

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[github-actions]

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[nyalldawson]

@alexbruy This is great! Thank you!

(thinking aloud) Some other ideas I've had for metadata related algorithms are:

• "Update layer metadata" -- copies all NON-EMPTY metadata fields from the source layer to the target layer, and leaves empty source fields unchanged in the target. (QgsAbstractMetadataBase::combine does this logic already)
• "Set metadata fields" -- exposes the simple metadata fields like title, type, language, abstract, identifier, ... (basically the ones which map across nicely to simple string, enum or datetime parameter types) so that the algorithm specifically sets the corresponding metadata value from the parameter value. This could have a choice for "Ignore empty fields", which would control whether a blank value in the parameter would be used to overwrite an existing value in the layer or not.
• "Add history metadata" -- exposes a string parameter for "history entry", and ADDS this to any existing history entries in the target layer

That'd address most of the metadata related processing needs. The missing remaining bit then would be some mechanism for auto-copying input layer metadata to generated layers, eg when running tools directly from the toolbox. I think to handle this we'd need API to control "input parameter to copy output layer metadata from" for each individual output that an algorithm generates, so that we can correctly copy the metadata from the correct source when an algorithm has multiple layer input parameters.

And then finally some mechanism to auto-append a history entry for generated layers summarising the processing tool and parameters used to generate that layer... 🥳

[alexbruy]

@nyalldawson I already was thinking about algorithm to combine metadata, your other suggestions are also valuable. I will add these new tools in the followup pull-request.

As for the API to add history entries to the metadata and copy metadata from input to output this would be really nice to have and if I recall correctly we even have a ticket about it.

[qgis-bot]

@alexbruy
A documentation ticket has been opened at qgis/QGIS-Documentation#9381
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!