Change diagnostic-id spelling to comply with other languages

[lmolkova]

.NET (track1, 2), Java, and Python ServiceBus and EventHub SDKs spell Diagnostic-Id, and java uses diagnostic-id which causes context propagation between different languages to break. Azure/azure-sdk-for-net#22775

Since tracing support is still in preview (both with the plugin and through application insights agent) and we presumably don't have a lot of customers using it, making behavior-breaking change in Java before GA is more preffereable than making all other languages support diagnostic-id spelling.

[alzimmermsft]

Do any of the other names need to be changed as well? This may run into RevApi issues as a public property is changing value.

[lmolkova]

Do any of the other names need to be changed as well?

nope, just this one, other names are not used beyond Java SDKs

This may run into RevApi issues as a public property is changing value.

Is there a way/review process to bypass it? Even though it's defined in azure-core, it's never used without tracing enabled

Yeah, this is what happened.

[alzimmermsft]

@lmolkova, this is achievable with a RevApi suppression, but before that let's see if @JonathanGiles and @srnagar are okay with this change.

[JonathanGiles]

Make the change!

[srnagar]

Given that this context is only used when tracing is enabled and the core-tracing library is still in preview, I am fine with making the change to this constant which actually fixes a bug.