fix(opentelemetry_cowboy): prevent telemetry handler crash on invalid HTTP/2 scheme

[velimir]

Cowboy does not validate the :scheme pseudo-header per RFC 9113, which states that scheme "is not restricted to http and https" and can be used for non-HTTP schemes via proxies/gateways. This means Cowboy passes through any value from the HTTP/2 HEADERS frame without validation.

When a malformed or unexpected scheme value reaches extract_scheme/2, the case clause crashes because it only handles <<"http">> and <<"https">>. This crash causes Erlang's telemetry library to permanently detach the handler for the affected node, resulting in all subsequent requests losing OpenTelemetry tracing until the node is restarted.

In production, this manifests as "almost all" requests suddenly missing trace IDs, with the root cause being a single malformed HTTP/2 request that crashed the telemetry handler.

The fix introduces a configurable valid_schemes option that maps scheme binaries to atoms. Unknown schemes now return undefined instead of crashing, which is consistent with how otel_http:extract_scheme/2 already handles unknown schemes from headers.

The default configuration preserves backwards compatibility by only accepting http and https. Users who need to support additional schemes (e.g., ws, wss) can extend the map via configuration.

Fixes: #567

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: velimir / name: Grigory Starinkin (71cca4b6cdefc16108ca2029046c1dd3c9993cf7, 9865d0618c5f33b1c02a01f9875ab259b6e99d6f)

[tsloughter]

Why was this closed?

[velimir]

Why was this closed?

GitHub automation in a private fork :/ it should've not been closed