Add total build time as metadata to artifacts

[dante858]

As it stands, buildstream doesn't contain the build time of an element in the resulting artifact. I believe including this would be useful information to keep track of.

[gtristan]

While I believe that the duration of a build of a given artifact can be observed in an artifact's build log by a human, it does seem interesting to have as machine readable metadata.

This may be powerful in conjunction with event reporting hooks: #188, such that CI builders may be able to trigger events when artifacts complete building and related infrastructure could notice which builds of which artifacts completed with which durations on which machines.