Closing open connections to Elasticsearch

We are using `elasticsearch-ruby` through the `chewy` gem, and we import documents in a Sidekiq worker. This means each Sidekiq thread that accesses Elasticsearch has its own instance of the `Elasticsearch::Client`, and when a (possibly unrelated) Sidekiq job fails, that worker thread is closed and a new thread is opened in the same process.

One thing we noticed though, is that the underlying Elasticsearch connections are only closed when Ruby's garbage collector collects the dead thread's `Elasticsearch::Client` instance, which seems to be the cause of a file descriptor leak in our application.

We think we have found a way to close these connections by adding the following code to the error handler in a custom Sidekiq middleware:
```ruby
Chewy.client.transport.transport.connections.each do |connection|
# This bit of code is tailored for the HTTPClient Faraday adapter
connection.connection.app.instance_variable_get(:@client)&.reset_all
end
```

However, this piece of code breaks multiple layers of abstractions, going through `chewy`, `elasticsearch`, `elasticsearch-transport`, `faraday` and `faraday-httpclient`, even accessing an otherwise unexposed instance variable at one point.

Is there a better way of closing connections to Elasticsearch? Are we missing something obvious about their lifecycle?

---

Digging into it, my understanding of the issue is that neither `elasticsearch` nor `elasticsearch-transport` provide a method to close connections.

It looks like `faraday` has `Faraday::Connection#close` but that appears to not actually be implemented in most adapters, and in particular not in the `faraday-httpclient` adapter that ends up being used in our app.

Of course, I may have missed something, and would be glad to know what if that's the case!