Handle "not modified" errors properly (#120)

For Connect RPCs that use GET and a "304 Not Modified" status
is returned, the metric will use the `http.status_code` attribute
with a value of 304, instead of an `rpc.connect.error_code`
attribute. (It would previously label the metric with an error
code of "unknown", which is misleading.)

Makefile

@@ -89,7 +89,7 @@ $(BIN)/license-header: Makefile
 
 $(BIN)/golangci-lint: Makefile
 	@mkdir -p $(@D)
-	GOBIN=$(abspath $(@D)) $(GO) install github.com/golangci/golangci-lint/cmd/golangci-lint@v1.50.0
+	GOBIN=$(abspath $(@D)) $(GO) install github.com/golangci/golangci-lint/cmd/golangci-lint@v1.51.0
 
 $(BIN)/protoc-gen-go: Makefile
 	@mkdir -p $(@D)

attributes.go

@@ -107,6 +107,12 @@ func statusCodeAttribute(protocol string, serverErr error) (attribute.KeyValue,
 		}
 		return codeKey.Int64(0), true // gRPC uses 0 for success
 	case connectProtocol:
+		if connect.IsNotModifiedError(serverErr) {
+			// A "not modified" error is special: it's code is technically "unknown" but
+			// it would be misleading to label it as an unknown error since it's not really
+			// an error, but rather a sentinel to trigger a "304 Not Modified" HTTP status.
+			return semconv.HTTPStatusCodeKey.Int(http.StatusNotModified), true
+		}
 		codeKey := attribute.Key("rpc." + protocol + ".error_code")
 		if serverErr != nil {
 			return codeKey.String(connect.CodeOf(serverErr).String()), true