Below is a list of some upcoming breaking changes for users to be aware of and to plan for, but not ones that are slated for the next release.
As of the 0.40 release, telemetry properties on logs now may be tagged, meaning the property value may have the shape
{ value: foo, tag: someString } instead of merely a primitive value. Unwrapped/untagged values are still supported.
See the updated type definition of ITelemetryProperties in @fluidframework/common-definitions v 0.21.
ITelemetryBaseLogger.send should be updated to handle these tagged values.
See this code
for an example of how to handle known tags, and update supportsTags to true when the logger properly handles tags.
Until then, in ITelemetryBaseLogger.send you may safely cast event: ITelemetryProperties to
{ [index: string]: TelemetryEventPropertyType; } and continue handling as-is,
since the FluidFramework will continue to handle tagged values itself unless supportsTags is true.