Rolling average work queue diagnostic (#24933).

Rolling average work queue diagnostic (#24933).

Background: https://github.com/dart-lang/sdk/issues/24933

Salient bits:

* sampling will only start if a client registers interest in diagnostics.
* sampling will stop if clients stop requesting diagnostics.
* sample results are coming back as a string (but a double would be better).
* the mapping of context folders to averages in not garbage collected; since it's only potentially leaking `_Average` objects (and not contexts) when folders are deleted, I think that's fairly benign.

BUG=24933
R=brianwilkerson@google.com, devoncarew@google.com

Committed: https://github.com/dart-lang/sdk/commit/c0466f28f51bc4e00f4646504e3736b3d9580eb2

[pquitslund, 2015-11-20 21:46:03]

pquitslund@google.com changed reviewers:
+ brianwilkerson@google.com, devoncarew@google.com

[devoncarew, 2015-11-20 22:17:06]

lgtm

Lgtm from the pov of the data being sent. Thanks!

https://codereview.chromium.org/1463923003/diff/1/pkg/analysis_server/lib/src...
File pkg/analysis_server/lib/src/domain_diagnostic.dart (right):

https://codereview.chromium.org/1463923003/diff/1/pkg/analysis_server/lib/src...
pkg/analysis_server/lib/src/domain_diagnostic.dart:148: void reset() {
Possibly resetTimerCountdown()? reset() is a bit ambiguous - whether it refers
to the timer or the running average.

[Brian Wilkerson, 2015-11-20 22:18:10]

> ... the mapping of context folders to averages in not garbage collected ...

If there isn't a TODO somewhere to fix this problem, please add one.

Other than that, LGTM

[pquitslund, 2015-11-20 22:22:38]

On 2015/11/20 22:18:10, Brian Wilkerson wrote:
> > ... the mapping of context folders to averages in not garbage collected ...
> 
> If there isn't a TODO somewhere to fix this problem, please add one.

Done!

> 
> Other than that, LGTM

[pquitslund, 2015-11-20 22:22:51]

Thanks!

https://codereview.chromium.org/1463923003/diff/1/pkg/analysis_server/lib/src...
File pkg/analysis_server/lib/src/domain_diagnostic.dart (right):

https://codereview.chromium.org/1463923003/diff/1/pkg/analysis_server/lib/src...
pkg/analysis_server/lib/src/domain_diagnostic.dart:148: void reset() {
On 2015/11/20 22:17:06, devoncarew wrote:
> Possibly resetTimerCountdown()? reset() is a bit ambiguous - whether it refers
> to the timer or the running average.

Done.

[pquitslund, 2015-11-20 22:24:55]

Committed patchset #2 (id:20001) manually as
c0466f28f51bc4e00f4646504e3736b3d9580eb2 (presubmit successful).