Change transport of direct client for customized MaxIdleConns

[xliuxu]

Currently we noticed the CPU usage for autoscaler is high when there are hundreds of pods running in cluster. From the CPU profling we can tell that it is spending on dialing for connections.

      flat  flat%   sum%        cum   cum%
         0     0%     0%      0.81s 19.61%  net/http.(*Transport).dialConn
         0     0%     0%      0.81s 19.61%  net/http.(*Transport).dialConnFor
         0     0%     0%      0.75s 18.16%  runtime.mcall
         0     0%     0%      0.74s 17.92%  net.(*Dialer).DialContext
         0     0%     0%      0.74s 17.92%  net/http.(*Transport).dial
     0.01s  0.24%  0.24%      0.74s 17.92%  runtime.schedule
         0     0%  0.24%      0.72s 17.43%  golang.org/x/sync/errgroup.(*Group).Go.func1
     0.70s 16.95% 17.19%      0.72s 17.43%  syscall.Syscall
         0     0% 17.19%      0.70s 16.95%  knative.dev/serving/pkg/autoscaler/metrics.(*serviceScraper).scrapePods.func1
         0     0% 17.19%      0.68s 16.46%  knative.dev/serving/pkg/autoscaler/metrics.(*httpScrapeClient).Scrape

The optimization in #8367 of @julz works but the default value 100 of MaxIdleConns is too small for pod scraping. The customized transport config from pkg/network with 1000 of MaxIdleConns should works better. After applying this change I see a significant CPU drop ( 54% -> 36%) with 300 pods running in cluster.

Release Note

None

[knative-prow-robot]

Welcome @lanceliuu! It looks like this is your first PR to knative/serving 🎉

[knative-prow-robot]

Hi @lanceliuu. Thanks for your PR.

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[markusthoemmes]

/ok-to-test

/hold
until after the release. Caution and stuff.

[vagababov]

/lgtm
/approve

[julz]

/lgtm

[markusthoemmes]

/lgtm
/approve
/unhold

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lanceliuu, markusthoemmes, vagababov

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