Expose prometheus generic builder.

So that people can easily try newer prometheus version
 in overlays:

 self: super: {
   prometheus_2 = (super.callPackage <nixpkgs/pkgs/servers/monitoring/prometheus> {}).generic {
     version = "2.8.1";
     sha256 = "0x8w0qdh4lcf19nmdlhvgzpy08c2a932d3k49cjwhi5npcsf858n";
     doCheck = false;
   };
 }

pkgs/servers/monitoring/prometheus/default.nix

@@ -2,8 +2,8 @@
 
 let
   goPackagePath = "github.com/prometheus/prometheus";
-
-  generic = { version, sha256, ... }@attrs:
+in rec {
+  generic = { version, sha256, doCheck ? true, ... }@attrs:
     let attrs' = builtins.removeAttrs attrs ["version" "sha256"]; in
       buildGoPackage ({
         name = "prometheus-${version}";
@@ -17,8 +17,6 @@ let
           inherit sha256;
         };
 
-        doCheck = true;
-
         buildFlagsArray = let t = "${goPackagePath}/vendor/github.com/prometheus/common/version"; in ''
           -ldflags=
              -X ${t}.Version=${version}
@@ -43,7 +41,7 @@ let
           platforms = platforms.unix;
         };
     } // attrs');
-in rec {
+
   prometheus_1 = generic {
     version = "1.8.2";
     sha256 = "088flpg3qgnj9afl9vbaa19v2s1d21yxy38nrlv5m7cxwy2pi5pv";