internal/chartconfig: add go env GOEXPERIMENT

This adds chart configuration for the go/goexperiment counters.

Because the underlying data collection (merged in cmd/go for over a
year) only tracks GOEXPERIMENT values that differ from the baseline,
the chart descriptions are aligned to reflect this implementation.
See CL 688135.

When performing cross-version aggregations, consumers of this data
must account for the go command's default behavior per version. An
experimental feature may be OFF by default in version X (meaning
telemetry tracks explicit opt-ins) but ON by default in version X+1
(meaning telemetry tracks explicit opt-outs).

Fixes golang/go#79212

Change-Id: Ic5b0096f55836a0aa5f9dff9b67ce923c0919d47
Reviewed-on: https://go-review.googlesource.com/c/telemetry/+/774780
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Hongxiang Jiang <hxjiang@golang.org>