cmd/go: rename flag counters and add buildmode values separately
Rename the subcommand flag counter names from
go/flag/<subcommand>/<flagname> to go/<subcommand>/flag/<flagname>.
Also remove the special case that adds counters for buildmode flag
values and instead add an additional counter for the flag values.
The new counter has the form go/<subcommand>/flag/buildmode:<flagvalue>.
We use a new CountFlagValue function that's been added to the
internal/telemetry package to help with this.
Finally add the go/invocations counter
Change-Id: I995b6b0009ba0f58faeb3e2a75f3b137e4136209
Reviewed-on: https://go-review.googlesource.com/c/go/+/583917
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Robert Findley <rfindley@google.com>