Reduce allocations to store config on context in Activator

[markusthoemmes]

Proposed Changes

The current store implementation has 5 allocations in its HTTP Middleware

• 2 in the call for r.WithContext(ctx)
• 1 to allocate the new value context
• 1 to allocate a new *Config
• 1 to load the untyped config

This reduces the footprint of the store to just one allocation by

1. Storing the intermediate Config until it needs to be updated.
2. Collapsing r.WithContext(ctx) calls with the context handler.

Release Note

NONE

/assign @julz @vagababov

[codecov]

Codecov Report

Merging #11013 (fe1c849) into main (b5d1348) will increase coverage by 0.05%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##             main   #11013      +/-   ##
==========================================
+ Coverage   87.95%   88.01%   +0.05%     
==========================================
  Files         188      189       +1     
  Lines        9116     9133      +17     
==========================================
+ Hits         8018     8038      +20     
+ Misses        842      839       -3     
  Partials      256      256              

Impacted Files	Coverage Δ
cmd/activator/main.go	0.00% <0.00%> (ø)
pkg/activator/config/store.go	100.00% <100.00%> (ø)
pkg/activator/handler/context_handler.go	96.15% <100.00%> (+0.32%)	⬆️
pkg/activator/net/revision_backends.go	91.32% <0.00%> (+0.91%)	⬆️

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[markusthoemmes]

/retest

HTTP2 probe stuff. Alerted the networking team.

[markusthoemmes]

With the benchmark in #11014 the results are as following:

benchmark                               old ns/op     new ns/op     delta
BenchmarkHandlerChain/sequential-16     21752         20881         -4.00%
BenchmarkHandlerChain/parallel-16       20981         20270         -3.39%

benchmark                               old allocs     new allocs     delta
BenchmarkHandlerChain/sequential-16     125            121            -3.20%
BenchmarkHandlerChain/parallel-16       125            121            -3.20%

benchmark                               old bytes     new bytes     delta
BenchmarkHandlerChain/sequential-16     7626          7145          -6.31%
BenchmarkHandlerChain/parallel-16       7801          7314          -6.24%

[vagababov]

/lgtm
/approve
👍

[julz]

spotted tiny nit, sorry; lgtm otherwise

[julz]

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: julz, vagababov

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