Single collection for multiple keys
The data you are generating from raspberry pi, temperature and humidity sensor is a time series data. MongoDB has documented some best practices and key tips for storing the time series data.

Time Series Data and MongoDB: Part 2 – Schema Design Best Practices

This explains the following schema patterns for time series data.

Scenario 1: One document per data point

Scenario 2: Time-based bucketing of one document per minute

Scenario 3: Size-based bucketing