Alright, first things first, let's get Firebase Firestore up and running for your project. Head over to the Firebase console and follow the steps to add Firebase to your project. Google's official documentation is super helpful here, so don't hesitate to check it out if you get stuck.

Now, let's talk about how to store your time-series data. You want to structure your data model in a way that makes querying by time a breeze. A straightforward approach is to use a timestamp as a unique document ID for each entry. This way, you can easily perform time-based queries.

Here's a simple structure for your data:

logs/{timestamp}
  log: ‘log message’
  severity: ‘log level’
  time: firestore.Timestamp

In this setup, 'logs' is a collection, and each document is a log entry with a timestamp as the document ID. The 'time' field captures when the log was created.

Firestore makes it pretty easy to add documents to a collection. Here's how you can add a log entry:

const logsCollection = firestore.collection('logs');

const timestamp = firestore.Timestamp.now();
const logEntry = {
  log: 'log message',
  severity: 'log level',
  time: timestamp,
};

logsCollection.doc(timestamp.toString()).set(logEntry);

This snippet creates a new document with the current timestamp as the ID and sets the fields with the log data.

Querying your data is where the magic happens. You can use Firestore's query options to fetch data within a specific time range. Here's an example:

const start = firestore.Timestamp.fromDate(new Date('2021-01-01'));
const end = firestore.Timestamp.fromDate(new Date('2022-01-01'));

logsCollection
  .orderBy('time')
  .startAt(start)
  .endAt(end)
  .get()
  .then((snapshot) => {
    snapshot.docs.forEach((doc) => {
      console.log(doc.data());
    });
  });

In this example, startAt() and endAt() define your time range. After calling get(), the snapshot contains all the documents that match your query. You can then loop through each document to access the log data.

Quick note: Firestore will prompt you to create an index to sort and filter on 'time'. Just follow the instructions in the Firebase console to set it up.

These steps should help you get started with using Firebase Firestore for storing and querying time-series data. Make sure your data model fits your needs and optimize your queries as necessary.