Override the metric map of each datasource as soon as it is created by
the manager. This override comes from a file whose path is provided by
a setting in config file.
Loading at creation time allows the correct datasource be used when
get_backend is called, this allows loading a datasource whose metric
names get updated outside the watcher's codebase.
The function 'load_metric_map' returns empty-dict in any error case.
Also in case the file is empty where safe_load is unable finds any
yaml documents, it will return None. [1]
Some minor refactoring in the test_manager file for readability and
added tests for file load and metric override.
1 - https://pyyaml.org/wiki/PyYAMLDocumentation
Change-Id: I1df16245f4c7dfd34066f3ab0553cd67154faa58
Implements: blueprint file-based-metric-map
Allows to define a global preference for metric datasources with the
ability for strategy specific overrides. In addition, strategies which
do not require datasources have the config options removed this is
done to prevent confusion.
Some documentation that details the inner workings of selecting
datasources is updated.
Imports for some files in watcher/common have been changed to resolve
circular dependencies and now match the overall method to import
configuration.
Addtional datasources will be retrieved by the manager if the
datasource throws an error.
Implements: blueprint global-datasource-preference
Change-Id: I6fc455b288e338c20d2c4cfec5a0c95350bebc36
