Fix duplicate error.type tags

[JamesNK]

Fixes #55159

Duplicate metrics tags can break some telemetry tooling. It's possible to have ASP.NET Core add multiple error.type tags to a counter. That's bad and should be fixed.

error.type can be added by exception middleware and it can be added in hosting if there is an unhandled exception. That means there can be duplicates if:

• Exception handling middleware handles an exception (tag added in middleware) and then there is an unhandled exception afterwards (tag added in hosting)
• Two configured exception handling middleware instances. An error is thrown and is handled (tag added in middleware 1) and then afterwards another error is thrown and handled by the second middleware (tag added in middleware 2).

Fix by conditionally adding error.type to tag collection only if it isn't present, aka first value wins.

Before (duplicate tags):

After (one tag):

[JamesNK]

/backport to release/8.0

[github-actions]

Started backporting to release/8.0: https://github.com/dotnet/aspnetcore/actions/runs/8792220010

[github-actions]

@JamesNK backporting to release/8.0 failed, the patch most likely resulted in conflicts:

$ git am --3way --ignore-whitespace --keep-non-patch changes.patch

Applying: Fix duplicate error.type tags
.git/rebase-apply/patch:351: new blank line at EOF.
+
warning: 1 line adds whitespace errors.
Using index info to reconstruct a base tree...
M	src/Hosting/Hosting/src/Internal/HostingMetrics.cs
M	src/Middleware/Diagnostics/test/FunctionalTests/TestFixture.cs
M	src/Middleware/Diagnostics/test/UnitTests/ExceptionHandlerTest.cs
Falling back to patching base and 3-way merge...
Auto-merging src/Middleware/Diagnostics/test/UnitTests/ExceptionHandlerTest.cs
CONFLICT (content): Merge conflict in src/Middleware/Diagnostics/test/UnitTests/ExceptionHandlerTest.cs
Auto-merging src/Middleware/Diagnostics/test/FunctionalTests/TestFixture.cs
Auto-merging src/Hosting/Hosting/src/Internal/HostingMetrics.cs
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Fix duplicate error.type tags
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!

[github-actions]

@JamesNK an error occurred while backporting to release/8.0, please check the run log for details!

Error: git am failed, most likely due to a merge conflict.