proxysql: use vendored prometheus-cpp

They specifically want v0.9.0 and already have it vendored. However,
we continue to use the civetweb from Nixpkgs. It may be necessary at
some point in the future to just allow them to use their vendored
sources for this altogether, but it works fine for now.

pkgs/servers/sql/proxysql/default.nix

@@ -24,7 +24,6 @@
 , openssl
 , pcre
 , perl
-, prometheus-cpp
 , python2
 , re2
 , zlib
@@ -109,7 +108,6 @@ stdenv.mkDerivation rec {
         { f = "libssl"; p = openssl; }
         { f = "lz4"; p = lz4; }
         { f = "pcre"; p = pcre; }
-        { f = "prometheus-cpp"; p = prometheus-cpp; }
         { f = "re2"; p = re2; }
     ]}
 
@@ -123,8 +121,9 @@ stdenv.mkDerivation rec {
     ln -s ${nlohmann_json.src}/single_include/nlohmann/json.hpp .
     popd
 
-    pushd prometheus-cpp/prometheus-cpp/3rdparty
-    replace_dep . "${civetweb.src}" civetweb
+    pushd prometheus-cpp
+    tar xf v0.9.0.tar.gz
+    replace_dep prometheus-cpp/3rdparty "${civetweb.src}" civetweb
     popd
 
     sed -i s_/usr/bin/env_${coreutils}/bin/env_g libssl/openssl/config