plugin/metadata: some cleanups (#1906)

* plugin/metadata: some cleanups

Name to provider.go as that's what being defined right now in the file.
Use request.Request because that's done in variables.go anyway. Name the
main storage M, because there is no further meaning behind.

Remove superfluous methods

Signed-off-by: Miek Gieben <miek@miek.nl>

* Fix test

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/metadata/provider_test.go

@@ -0,0 +1,48 @@
+package metadata
+
+import (
+	"context"
+	"reflect"
+	"testing"
+)
+
+func TestMD(t *testing.T) {
+	tests := []struct {
+		addValues      map[string]interface{}
+		expectedValues map[string]interface{}
+	}{
+		{
+			// Add initial metadata key/vals
+			map[string]interface{}{"key1": "val1", "key2": 2},
+			map[string]interface{}{"key1": "val1", "key2": 2},
+		},
+		{
+			// Add additional key/vals.
+			map[string]interface{}{"key3": 3, "key4": 4.5},
+			map[string]interface{}{"key1": "val1", "key2": 2, "key3": 3, "key4": 4.5},
+		},
+	}
+
+	// Using one same md and ctx for all test cases
+	ctx := context.TODO()
+	ctx = context.WithValue(ctx, metadataKey{}, M{})
+	m, _ := FromContext(ctx)
+
+	for i, tc := range tests {
+		for k, v := range tc.addValues {
+			m.SetValue(k, v)
+		}
+		if !reflect.DeepEqual(tc.expectedValues, map[string]interface{}(m)) {
+			t.Errorf("Test %d: Expected %v but got %v", i, tc.expectedValues, m)
+		}
+
+		// Make sure that md is recieved from context successfullly
+		mFromContext, ok := FromContext(ctx)
+		if !ok {
+			t.Errorf("Test %d: md is not recieved from the context", i)
+		}
+		if !reflect.DeepEqual(m, mFromContext) {
+			t.Errorf("Test %d: md recieved from context differs from initial. Initial: %v, from context: %v", i, m, mFromContext)
+		}
+	}
+}