Collapse revision context into specialized context to avoid allocations

[markusthoemmes]

Proposed Changes

Always wanted to play with custom value contexts, so here we are! This spares us a bit of linked list lookup and context allocation, so yay!

benchmark                                                         old ns/op     new ns/op     delta
BenchmarkContextHandler/context_handler_success-sequential-16     923           841           -8.88%
BenchmarkContextHandler/context_handler_success-parallel-16       176           166           -5.68%
BenchmarkContextHandler/context_handler_failure-sequential-16     15401         11938         -22.49%
BenchmarkContextHandler/context_handler_failure-parallel-16       11216         11829         +5.47%

benchmark                                                         old allocs     new allocs     delta
BenchmarkContextHandler/context_handler_success-sequential-16     7              6              -14.29%
BenchmarkContextHandler/context_handler_success-parallel-16       7              6              -14.29%
BenchmarkContextHandler/context_handler_failure-sequential-16     38             38             +0.00%
BenchmarkContextHandler/context_handler_failure-parallel-16       38             38             +0.00%

benchmark                                                         old bytes     new bytes     delta
BenchmarkContextHandler/context_handler_success-sequential-16     592           560           -5.41%
BenchmarkContextHandler/context_handler_success-parallel-16       592           560           -5.41%
BenchmarkContextHandler/context_handler_failure-sequential-16     3944          4111          +4.23%
BenchmarkContextHandler/context_handler_failure-parallel-16       3663          3942          +7.62%

Release Note

NONE

/assign @julz @vagababov

[codecov]

Codecov Report

Merging #11009 (df06d78) into main (4e5fde5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main   #11009   +/-   ##
=======================================
  Coverage   87.97%   87.97%           
=======================================
  Files         188      188           
  Lines        9115     9116    +1     
=======================================
+ Hits         8019     8020    +1     
- Misses        840      841    +1     
+ Partials      256      255    -1     

Impacted Files	Coverage Δ
pkg/activator/handler/context.go	100.00% <100.00%> (ø)
pkg/activator/handler/context_handler.go	95.83% <100.00%> (-0.17%)	⬇️
pkg/activator/net/revision_backends.go	90.41% <0.00%> (-0.92%)	⬇️
pkg/autoscaler/statforwarder/leases.go	78.41% <0.00%> (+1.43%)	⬆️

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[julz]

👍

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: julz

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