[sdk-platform-java] [o11y] Investigate 500 error reported when client points to non-existent host

**HTTP Client failures case name and attributes conform to requirements**

* **Manual test:** We try to hit a non-existent host.

```java
TranslationServiceSettings.Builder settingsBuilder = TranslationServiceSettings.newBuilder();

settingsBuilder
.getStubSettingsBuilder()
.setEndpoint("non-existent:1234")
.setTracerFactory(createOpenTelemetryTracerFactory());
```

* **HTTP**
*

* Confirmed this 500 (`UnknwonException`) is part of unwrapping the exception (see [related code](https://github.com/googleapis/google-cloud-java/blob/59441275c916aa4698463c116de6f80558f46534/sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/tracing/ErrorTypeUtil.java#L204-L212)). The actual code may not be desired but it respects the logic, so if a fix is wanted it should be done in gax-httpjson