Option to manage concurrency

[zerkms]

I monitor (using a home-made script) quite a few hostnames (over a 100), and planning to switch to this exporter instead.

With the current implementation prometheus would be request a lot of them once.

How about having some --concurrency-level parameter that limits number of concurrently run ssllabs inspections?

And the excessive requests may wait for a concurrency token (and time out, if they didn't obtain it).

[anas-aso]

@zerkms what can of issues did you face with the current implementation ? I am running this exporter with many hosts and I didn't face any issues so far.
Did you try scaling the exporter horizontally ? I simply add more replicas if there are more targets to assess by Prometheus this way each exporter instance would still run a reasonable amount of assessments in parallel.

[zerkms]

The intention is to remove some extra pressure for ssllabs service:

Keep down the number of concurrent assessments to a minimum. If you're not in a hurry, test only one hostname at a time.

https://github.com/ssllabs/ssllabs-scan/blob/master/ssllabs-api-docs-v3.md#access-rate-and-rate-limiting