plugin/metadata: add metadata plugin (#1894)

* plugin/metadata: add metadata plugin

* plugin/metadata: Add MD struct, refactor code, fix doc

* plugin/metadata: simplify metadata key

* plugin/metadata: improve setup_test

* Support of metadata by rewrite plugin. Move calculated variables to metadata.

* Move variables from metadata to pkg, add UTs, READMEs change, metadata small fixes

* Add client port validation to variables_test

* plugin/metadata: improve README

* plugin/metadata: rename methods

* plugin/metadata: Update Metadataer interface, update doc, cosmetic code changes

* plugin/metadata: move colllisions check to OnStartup(). Fix default variables metadataer.

* plugin/metadata: Fix comment for method setValue

* plugin/metadata: change variables order to fix linter warning

* plugin/metadata: rename Metadataer to Provider

plugin/metadata/metadataer_test.go

@@ -0,0 +1,47 @@
+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()
+	md, ctx := newMD(ctx)
+
+	for i, tc := range tests {
+		for k, v := range tc.addValues {
+			md.setValue(k, v)
+		}
+		if !reflect.DeepEqual(tc.expectedValues, map[string]interface{}(md)) {
+			t.Errorf("Test %d: Expected %v but got %v", i, tc.expectedValues, md)
+		}
+
+		// Make sure that MD is recieved from context successfullly
+		mdFromContext, ok := FromContext(ctx)
+		if !ok {
+			t.Errorf("Test %d: MD is not recieved from the context", i)
+		}
+		if !reflect.DeepEqual(md, mdFromContext) {
+			t.Errorf("Test %d: MD recieved from context differs from initial. Initial: %v, from context: %v", i, md, mdFromContext)
+		}
+	}
+}