fix(): build error in certain environments

[skahack]

Fixed an error that occurred when apollo-server-express did not exist as a dependency.

node_modules/@bmatei/apollo-prometheus-exporter/hooks.js:7:47: error: Could not resolve "apollo-server-express/package.json" (mark it as external to exclude it from the bundle, or surround it with try/catch to handle the failure at run-time)
    7 │ const package_json_1 = __importDefault(require("apollo-server-express/package.json"));
      ╵                                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1 error