The metricsEndpoint variable is typed as 'RouteOptions | string | null' but is only assigned either a RouteOptions object or a string. The 'null' type is never used and should be removed from the type annotation to accurately reflect the actual possible values.

Since opts.prometheusKey, is nullable, so is the route.

I see that the entire statement is always non-null:

  const metricsEndpoint: RouteOptions | string | null = opts.prometheusKey
    ? {
        url: "/metrics",
        method: "GET",
        handler: async () => {}, // Overridden by fastify-metrics
        onRequest: async (request: FastifyRequest, reply: FastifyReply) => {
          if (
            request.headers.authorization !== `Bearer ${opts.prometheusKey}`
          ) {
            reply.code(401).send({ status: "error", message: "Unauthorized" });
            return;
          }
        },
      }
    : "/metrics";

If opts.prometheusKey is null then it goes to the falsy branch and evaluates to "/metrics". Maybe it's not what you intended?

The Loki transport configuration modifies the options object at module level before the app is created. If the logger option is a boolean (true/false) rather than an object, the code in the else block (lines 111-114) will override it, potentially causing unexpected behavior. The condition on line 93 should also check if existingLogger is not a boolean value to properly handle all logger configuration types.

For the above comment, you can proceed as follows if the type is a boolean:

• If the boolean is true, you existing logic should already work properly.
• If false then you might not want to enable loki.