Enhanced Metric Tags

The existing set of tags that have been available on metrics was a simple mapping of string values to single string keys. Some sources, however, may produce metrics that contain tags that don’t fit into this model, such as tags that are bare strings (rather than a key/value pair) or have multiple values for a given key.

With this release, tag values for a given key may now contain a set of values, each of which may be a string value or null, which represents a “bare” tag (i.e. a tag with no value, as distinct from an empty string value). This new tag set only stores unique values, as duplicated tags are not useful for any component, and so each tag name/value pair will appear only once across all tags.

For compatibility with previous releases, this value set also tracks the last seen or assigned value for each tag. Since most components, in particular sinks, can only make use of a single value for each tag, this last value is used by those components.

The scripting components of lua and remap have been extended with a configuration option of metric_tag_values. This controls how these tag values are exposed to scripts. In the default setting of single, tag values will be exposed as the single value described above. This behavior matches the existing behavior of scripts so that no script changes will be needed. When set to full, however, all tag values are exposed as an array for each element. In either case, scripts may assign either a single value or an array of values to a tag.

For example:

.tags.host = "localhost" # Assign a single string value
.tags.bare = null # Create a single-valued bare tag
.tags.complex = ["remotehost", null, "otherhost"] # Creates three tag values

This metric_tag_values setting also shows up in the codec configuration of sinks and controls how metric tags are exposed in codecs that can encode metrics. As above, when set to full, these codecs will expose all values of tags either as arrays (for the json codec) or as repeated instances of each tag (for the text codec). This setting does not apply to the native and native_json codecs which always expose all tag values in order to seamlessly transport the values to other Vector instances.