Advanced Configuration

This commit introduces advanced configuration.  The rate-interval and
label-prefix flags are removed, and replaced by a configuration file
that allows you to specify series queries and the rules for transforming
those into metrics queries and API resources.

pkg/config/loader.go

@@ -0,0 +1,32 @@
+package config
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+
+	yaml "gopkg.in/yaml.v2"
+)
+
+// FromFile loads the configuration from a particular file.
+func FromFile(filename string) (*MetricsDiscoveryConfig, error) {
+	file, err := os.Open(filename)
+	defer file.Close()
+	if err != nil {
+		return nil, fmt.Errorf("unable to load metrics discovery config file: %v", err)
+	}
+	contents, err := ioutil.ReadAll(file)
+	if err != nil {
+		return nil, fmt.Errorf("unable to load metrics discovery config file: %v", err)
+	}
+	return FromYAML(contents)
+}
+
+// FromYAML loads the configuration from a blob of YAML.
+func FromYAML(contents []byte) (*MetricsDiscoveryConfig, error) {
+	var cfg MetricsDiscoveryConfig
+	if err := yaml.Unmarshal(contents, &cfg); err != nil {
+		return nil, fmt.Errorf("unable to parse metrics discovery config: %v", err)
+	}
+	return &cfg, nil
+}