Allow restricting histogram keys to a known set in Histograms.json

[Frank Bertsch]

Georg, what did you mean by "Without using a keyed histogram, the label
definition would be repeated over multiple histogram definitions."?


On 3/2/17 08:18, Georg Fritzsche wrote:
> Originally we added keyed histograms to allow using string keys that
> were not all known before.
> However, we have use-cases were it makes sense to use the two-level
> keyed histogram structure (name, key), even when the set of keys is
> known before (e.g. to share common definitions).
>
> So in bug 1343855
> <https://bugzilla.mozilla.org/show_bug.cgi?id=1343855> we plan to add
> an additional optional field "keys" for keyed histograms that allows
> specifying the valid key strings.
> Any recording to the histogram with an invalid key string will be
> ignored and a warning/error be print
>
> E.g. consider:
> "STORAGE_UPDATE_RESULT": {
>   "keyed": true,
>   "kind": "categorical",
>   "keys": ["blocklists.addons", "blocklists.certificates",
> "pinning.pins"],
>   "labels": ["success", "backoff", "network_error", "server_error",
> "sign_error"],
>   ...
> }
>
> Without using a keyed histogram, the label definition would be
> repeated over multiple histogram definitions.
>
> Feedback, concerns?
>
> Georg
>
>
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