Add a standard version semantic attribute.
When creating trace data or metrics, it can be extremely useful to know the specific version that
emitted the iota of span or measurement being viewed. However, versions can mean different things
to different systems and users. In addition, downstream analysis systems may wish to expose
functionality related to the type of a version (such as detecting when versions are newer or older).
To support this, we should standardize a version attribute with optional hints as to the type of the
version.
A version is a semantic attribute that can be applied to other resources, such as Service,
Component, Library, Device, Platform, etc. A version attribute is optional, but recommended.
The definition of a version is a key-value attribute pair of string to string, with naming schemas
available to hint at the type of a version, such as the following:
version=semver:1.2.3 (a semantic version)
version=git:8ae73a (a git sha hash)
version=0.0.4.2.20190921 (a untyped version)
Since this is just an attribute pair, no special handling is required, although SDKs may provide helper methods to construct schema-appropriate values.
Tagging service resources with their version is generally suggested by analysis tools -- see JAEGER_TAGS for an example -- but lacks standardization.