feat: support explain operations in CommandOperationV2

This adds an extra field to the `CommandOperationV2` base class
that informs command construction to support explain operations.

lib/operations/command_v2.js

@@ -8,9 +8,10 @@ class CommandOperationV2 extends OperationBase {
     super(options);
 
     this.ns = parent.s.namespace.withCollection('$cmd');
-    this.readPreference = resolveReadPreference(parent, options);
-    this.readConcern = resolveReadConcern(parent, options);
-    this.writeConcern = resolveWriteConcern(parent, options);
+    this.readPreference = resolveReadPreference(parent, this.options);
+    this.readConcern = resolveReadConcern(parent, this.options);
+    this.writeConcern = resolveWriteConcern(parent, this.options);
+    this.explain = false;
 
     // TODO(NODE-2056): make logger another "inheritable" property
     if (parent.s.logger) {
@@ -25,14 +26,19 @@ class CommandOperationV2 extends OperationBase {
       this.logger.debug(`executing command ${JSON.stringify(cmd)} against ${this.ns}`);
     }
 
+    let fullResponse = this.options.full;
+    if (this.explain) {
+      cmd = { explain: cmd };
+      fullResponse = false;
+    }
+
     server.command(this.ns.toString(), cmd, this.options, (err, result) => {
       if (err) {
         callback(err, null);
         return;
       }
 
-      // full response was requested
-      if (this.options.full) {
+      if (fullResponse) {
         callback(null, result);
         return;
       }