Nodejs can't get post request body

I created nodejs api and react app and I dockerized them.

When I try to send a post request through api, the request body getting empty.

I created api docker container like this:

FROM node:lts
# Create app directory
WORKDIR /app
# Install app dependencies
COPY package*.json ./

RUN npm install
# Copy app source code
COPY . .

#Expose port and start application
EXPOSE 4000
CMD [ "npm", "start" ]

I also created react app container like this:

FROM node:latest
WORKDIR /app
COPY package.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["npm", "start"]

I double checked to express json middleware in nodejs:

app.use(express.urlencoded());
app.use(express.json());

I am sending post request like this in react app:

fetch(`${serviceUrl}/metrics`, {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
  },
  body: JSON.stringify(getPerfanalytics().values),
})

I can see that request is working good but the request body is empty always.

Here is how can I get post request:

    export const saveMetric = (req, res) => {
        let params = req.body;
        console.log(params);
        if (typeof params === 'string') {
            params = JSON.parse(params)
        }
        new Metric(params).save().then(metric => {
            res.json({
                message: 'Metric saved',
                data: metric
            });
        }).catch((err) => {
            res.status(500);
            res.json({
                message: 'Metric saving error',
                error: err
            });
        }

);
}

How can I solve this problem?

Thanks