Allow atom :all as :tags option

[hauleth]

This is meant for collectors to just use metadata as a whole. The reasoning for that is that in many cases user may know, that all provided metadata will be relevant (especially true for metrics of the current application). Currently the way to extract relevant metrics is to use Map.take/2, but that can introduce unwanted slowdown due to requirement of constructing new map even if all keys are selected.

In my benchmarking of my project Map.take/2 (internal Map.take/3 function to be exact) was responsible for 7.83% of the whole runtime (very tight loop), and I simply take whole metadata into account, so that additional processing is not needed.

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@@             Coverage Diff             @@
##              main     #126      +/-   ##
===========================================
- Coverage   100.00%   99.34%   -0.66%     
===========================================
  Files            2        2              
  Lines          145      152       +7     
===========================================
+ Hits           145      151       +6     
- Misses           0        1       +1     

Files with missing lines	Coverage Δ
lib/telemetry_metrics.ex	98.98% <100.00%> (-1.02%)	⬇️

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[josevalim]

I wonder if there are other ways to optimize this. The issue with adding :all is that we now need to change all formatters to support it, including the ConsoleFormatter here. So if we are going down this route, I'd rather allow :tags to be a one-arity function, that receives all metadata and returns the metadata we want as tags, this would allow us to deprecate tag_values too.

[josevalim]

Actually, I think in many cases projects like Phoenix includes conn and socket as metadata, so using :all will be problematic as it either won't work OR it will cause huge amounts of data to be exported.

[hauleth]

@josevalim yeah, but the idea there was to not use that as a default but as an optimisation for places where it really is a problem. That way existing metrics gatherers would still work perfectly fine, it was just for that 1% of the cases where that additional function call does matter.

[josevalim]

So let's go with a function and deprecate tag_values, because that would allow you to optimize all cases you care about, since you could also rewrite:

tags: [:foo, :bar]

as:

tags: fn %{foo: foo, bar: bar} -> %{foo: foo, bar: bar} end

WDYT?

[hauleth]

That can be a solution as well. The question is whether we should do the translation to function in Telemetry.Metrics or just add new possible value there?

[josevalim]

Doing the translation would be a breaking change, so we need add a new value for formatters to handle.

[josevalim]

We can do the translation in the future. So if we deprecate tag_values now, we will force formatters to upgrade, and then we translate it in a future release.