Just to understand, are we talking about exposing some #contextes, or their memory size?
I'd say:
- definitely do the work to expose that as a column in memory-infra. The code that dumps v8 stats is in src/gin/v8_isolate_memory_dump_provider.cc . 
Once that is done there are two ways:
A: manually plumbing that all the way through chrome_metrics_emitter.cc (similarly to https://chromium-review.googlesource.com/c/808445/)
B: wait for lalitm@ to finish his work (eta this Q) to get the full memory graph available to chrome_metrics_emitter.cc. That would reduce the amount of changes to few lines in chrome_metrics_emitter.cc, and avoid all the rest of the boilerplate that you see in the CL linked in A.

In both cases, it seems that any addition to UKM requires a re-iteration on the UKM collection doc (e.g. go/rdjvy is what I did last time). It's usually never a problem and a matter of few mins, but takes at least a week until gets through the queue of the people who can stamp it. So I'd start doing that in parallel to avoid bubbles in the pipeline.

Thanks for the detailed answer, primiano@!

I'll plumb src/gin/v8_isolate_memory_dump_provider.cc and rely on lalitm@ work to pick up the new data in chrome_metrics_emitter.cc. ETA this Q works well for us.

lalitm@, is there a bug I could subscribe to for updates?

> Just to understand, are we talking about exposing some #contextes, or their memory size?
It the number of contexts. We do not know memory size per context (due to huge runtime overhead of tracking it). Also there is object sharing between contexts.

> In both cases, it seems that any addition to UKM requires a re-iteration on the UKM collection doc
I'll create a copy of the document, add new items, and send out for review as "December edition".

crbug/728199 is the one I will be using for all metrics related memory graph work!

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/298f0cd4389654e36a878410fe5ff92da255e32a

commit 298f0cd4389654e36a878410fe5ff92da255e32a
Author: Ulan Degenbaev <ulan@chromium.org>
Date: Mon Dec 11 18:14:31 2017

[heap] Provide the number of native and detached context via Heap API.

This adds two fields to the HeapStatistics struct:
- number_of_native_contexts,
- number_of_detached_contexts.

Bug: chromium:793789
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: If6942a97fd22a9e70781eed2aa286aba4c0e7f70
Reviewed-on: https://chromium-review.googlesource.com/819730
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50007}
[modify] https://crrev.com/298f0cd4389654e36a878410fe5ff92da255e32a/include/v8.h
[modify] https://crrev.com/298f0cd4389654e36a878410fe5ff92da255e32a/src/api.cc
[modify] https://crrev.com/298f0cd4389654e36a878410fe5ff92da255e32a/src/heap/heap.cc
[modify] https://crrev.com/298f0cd4389654e36a878410fe5ff92da255e32a/src/heap/heap.h
[modify] https://crrev.com/298f0cd4389654e36a878410fe5ff92da255e32a/test/cctest/test-api.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bfe58719079deaff42e43116fdc9735881837cc2

commit bfe58719079deaff42e43116fdc9735881837cc2
Author: Ulan Degenbaev <ulan@chromium.org>
Date: Fri Dec 15 19:40:54 2017

[tracing] Dump context counts in v8 memory dump provider.

This patch adds the following dumps in v8 memory dump provider:
- v8/isolate/contexts/detached_context
- v8/isolate/contexts/native_context
Both dumps contain an "object_count" scalar value.

The value of detached_context is the number of contexts that were
detached (due to iframe navigation or removal) and not yet garbage
collected. This number being non-zero indicates potential memory leak.

The value of native_context is the number of the top-level contexts
created for the main window and iframes. Increase of this number
over time indicates a memory leak.

Bug: 793789
Change-Id: I4753454747c398fac4fc153fcccb1288d28f2a2d
Reviewed-on: https://chromium-review.googlesource.com/822336
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524438}
[modify] https://crrev.com/bfe58719079deaff42e43116fdc9735881837cc2/base/trace_event/memory_infra_background_whitelist.cc
[modify] https://crrev.com/bfe58719079deaff42e43116fdc9735881837cc2/gin/v8_isolate_memory_dump_provider.cc
[modify] https://crrev.com/bfe58719079deaff42e43116fdc9735881837cc2/gin/v8_isolate_memory_dump_provider_unittest.cc