Splitting out the misc client into separate clients as per current documentation (#2574)

Octokit.Reactive/Clients/IObservableMetaClient.cs

@@ -0,0 +1,20 @@
+using System;
+
+namespace Octokit.Reactive
+{
+    /// <summary>
+    /// A client for GitHub's meta APIs.
+    /// </summary>
+    /// <remarks>
+    /// See the <a href="https://docs.github.com/rest/meta">Meta API documentation</a> for more details.
+    /// </remarks>
+    public interface IObservableMetaClient
+    {
+        /// <summary>
+        /// Retrieves information about GitHub.com, the service or a GitHub Enterprise installation.
+        /// </summary>
+        /// <exception cref="ApiException">Thrown when a general API error occurs.</exception>
+        /// <returns>An <see cref="Meta"/> containing metadata about the GitHub instance.</returns>
+        IObservable<Meta> GetMetadata();
+    }
+}