Commits on Oct 4, 2024

1. implement OpenMetrics support for real …

   I discovered that thanos-federate-proxy lies about its
   capabilities: it's not truly OpenMetrics compliant.
   
   The prometheus metrics scrape endpoint representation
   differs from OpenMetrics in subtle ways. This makes consumers
   that only support OpenMetrics fail to scrape anything produced
   by thanos-federate-proxy.
   
   I don't think this is a good idea, but I wanted to see if it was
   technically possible. I also don't know if this is a dumb
   contortion or if there is a better way to achieve it.
   
   thanos-federate-proxy retrieves from the upstream using
   Prometheus Query API. This API returns `model.Value`, which
   is a wire representation that does not have all the same information
   presented in a scrape endpoint (MetricsFamily). If we transformed
   to MetricsFamily _somehow_, we would be able to use the `expfmt`
   package from `prometheus/common` module that has the official bits
   for content negotiation and serialization into the
   scrape endpoint textual representation.
   
   So the challenge here
   was how to transform `model.Value` to MetricsFamily.
   I discovered Prometheus offers Metadata API, which gives us the missing
   bits. Unfortunately, Thanos does not expose this API, so this commit
   requires you to wire up the metadata API through the upstream prometheus.
   API Queries will hit Thanos, and metadata API calls will hit prometheus.
   The latter is done outside the critical path on an interval, which is currently
   hardcoded to 1 minute.
   
   No guarantees here. I'm not sure if the transformation is done correctly.

   

   vroldanbet committed Oct 4, 2024
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