SSE: Support for ML query node (#69963)

* introduce a new node-type ML and implement a command outlier that uses ML plugin as a source of data.
* add feature flag mlExpressions that guards the feature

pkg/expr/ml/node.go

@@ -0,0 +1,63 @@
+package ml
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend"
+	jsoniter "github.com/json-iterator/go"
+
+	"github.com/grafana/grafana/pkg/api/response"
+)
+
+var json = jsoniter.ConfigCompatibleWithStandardLibrary
+
+type CommandType string
+
+const (
+	Outlier CommandType = "outlier"
+
+	// format of the time used by outlier API
+	timeFormat = "2006-01-02T15:04:05.999999999"
+
+	defaultInterval = 1000 * time.Millisecond
+)
+
+// Command is an interface implemented by all Machine Learning commands that can be executed against ML API.
+type Command interface {
+	// DatasourceUID returns UID of a data source that is used by machine learning as the source of data
+	DatasourceUID() string
+	// Execute creates a payload send request to the ML API by calling the function argument sendRequest, and then parses response.
+	// Function sendRequest is supposed to abstract the client configuration such creating http request, adding authorization parameters, host etc.
+	Execute(from, to time.Time, sendRequest func(method string, path string, payload []byte) (response.Response, error)) (*backend.QueryDataResponse, error)
+}
+
+// UnmarshalCommand parses a config parameters and creates a command. Requires key `type` to be specified.
+// Based on the value of `type` field it parses a Command
+func UnmarshalCommand(query []byte, appURL string) (Command, error) {
+	var expr CommandConfiguration
+	err := json.Unmarshal(query, &expr)
+	if err != nil {
+		return nil, fmt.Errorf("failed to unmarshall Machine learning command: %w", err)
+	}
+	if len(expr.Type) == 0 {
+		return nil, fmt.Errorf("required field 'type' is not specified or empty.  Should be one of [%s]", Outlier)
+	}
+
+	if len(expr.Config) == 0 {
+		return nil, fmt.Errorf("required field 'config' is not specified")
+	}
+
+	var cmd Command
+	switch mlType := strings.ToLower(expr.Type); mlType {
+	case string(Outlier):
+		cmd, err = unmarshalOutlierCommand(expr, appURL)
+	default:
+		return nil, fmt.Errorf("unsupported command type. Should be one of [%s]", Outlier)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("failed to unmarshal Machine learning %s command: %w", expr.Type, err)
+	}
+	return cmd, nil
+}