Export Application Insights connection string, not only instrumentation key

From the docs:

On March 31, 2025, support for instrumentation key ingestion will end. Instrumentation key ingestion will continue to work, but we'll no longer provide updates or support for the feature. Transition to connection strings to take advantage of new capabilities.

Currently, we only export the instrumentation key as a Key Vault secret. A better practice would be to export the connection string and eventually retire the instrumentation key.

Some services that want to use connection strings even try to craft it from the instrumentation key.

MR: !890 (merged)