Change heartbeat to use connection strings

Change implementation of heartbeat to use connection strings instead of instrumentation key, as technical support for instrumentation key- based data ingestion will end.

Current implementation will continue to work for some time.

Ref: https://learn.microsoft.com/en-us/azure/azure-monitor/app/connection-strings

Locate the heartbeat implementation and trace how it currently consumes the instrumentation key. Read the linked Azure Monitor connection-string guidance, then update the heartbeat path to use connection strings and verify that heartbeat ingestion still works without the instrumentation key.

Written by the indexing model from the issue text.