analysis/appengine/template: truncate long label values
analysis/appengine/template: truncate long label values

Change-Id: Ica888f19254759a060015e9635cffd0231721dd4
Reviewed-on: https://go-review.googlesource.com/35946
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/perf,golang/perf