Add metal-cli

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Fixes equinix/metal-cli#38

Based on https://github.com/goreleaser/homebrew-tap/blob/master/Formula/goreleaser.rb

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This draft represents my first guess at what this configuration will look like. I haven't found supported examples to demonstrate how a single binary should be fetched and then renamed at install. Examples that I have found expect a tar ball.

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It seems that a tar ball may be needed.

Alternatively, we could take the build approach. I do not know if goreleaser handled updates to this package format (build vs binary):

diff --git a/Formula/metal-cli.rb b/Formula/metal-cli.rb
index c403353..f792068 100644
--- a/Formula/metal-cli.rb
+++ b/Formula/metal-cli.rb
@@ -2,43 +2,21 @@
 # frozen_string_literal: true
 
 # This file was generated by GoReleaser. DO NOT EDIT.
-class MetalCLI < Formula
+class MetalCli < Formula
   desc "Equinix Metal CLI"
   homepage "https://githubcom/equinix/metal-cli"
   version "0.6.0"
   license "MIT"
+  url "https://api.github.com/repos/equinix/metal-cli/tarball/v0.6.0"
 
-  on_macos do
-    if Hardware::CPU.intel?
-      url "https://github.com/equinix/metal-cli/releases/download/v0.6.0/metal-darwin-amd64"
-      sha256 "ebcb8bd18e83bd39a6a6cf711802654b349f83c7f75de27e1113b704d9682315"
-      def install
-        bin.install "metal-darwin-amd64" => "metal"
-      end
-    end
-  end
+  depends_on "go" => :build
 
-  on_linux do
-    if Hardware::CPU.arm? && !Hardware::CPU.is_64_bit?
-      url "https://github.com/equinix/metal-cli/releases/download/v0.6.0/metal-linux-armv6"
-      sha256 "67f8aff5a2fdc156a192125f0b2d9cd73208da5a764b71b247f41c996d13e8b2"
-      def install
-        bin.install "metal-linux-armv6" => "metal"
-      end
-    end
-    if Hardware::CPU.intel?
-      url "https://github.com/equinix/metal-cli/releases/download/v0.6.0/metal-linux-amd64"
-      sha256 "50f575b8e5355cd603df3bcae4ceac30fe4fdec63da61d2fb06f02163fb267d2"
-      def install
-        bin.install "metal-linux-amd64" => "metal"
-      end
-    end
-    if Hardware::CPU.arm? && Hardware::CPU.is_64_bit?
-      url "https://github.com/equinix/metal-cli/releases/download/v0.6.0/metal-linux-arm64"
-      sha256 "36797022316a2fc62db884170d2946eef9022848aa5540419b08d8d0d25e1569"
-      def install
-        bin.install "metal-linux-arm64" => "metal"
-      end
+  def install
+    ENV["GOPATH"] = HOMEBREW_CACHE/"go_cache"
+    (buildpath/"src/github.com/equinix/metal-cli").install buildpath.children
+
+    cd "src/github.com/equinix/metal-cli" do
+      system "go", "build", "-o", bin/"metal", "cmd/metal/main.go"
     end
   end