[release/8.0] Fix duplicate error.type tags in metrics

[JamesNK]

Fix duplicate error.type tags in metrics

Description

ASP.NET Core has a prominent metric - http.request.server.duration - that reports information about HTTP request duration along with important metadata. One piece of metadata is whether an error occurred during the request. This is the error.type tag.

The error.type tag can be reported from multiple places, and it's possible to get duplicate tag values in rare situations. Duplicate metrics tags can break some telemetry tooling.

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.

Fixes #55159

Customer Impact

Reported by customer at #55159. Prometheus (popular OSS telemetry database) can't handle duplicates and fails to load new metrics.

There is also open-telemetry/opentelemetry-dotnet#5199 which appears to be from the same issue and has 6 +1 votes.

A workaround is to not use the ASP.NET Core error middleware, but it is a very popular feature, and it's not obvious that it is the fix.

Regression?

• Yes
• No

Risk

• High
• Medium
• Low

The fix is simple: don't add error.type tag to metrics collection if it is already present.

Verification

• Manual (required)
• Automated

Before:

After:

Packaging changes reviewed?

• Yes
• No
• N/A

[JamesNK]

Email approval