http: add jitter support for max_connection_duration

[Retr0-XD]

Description

Fixes #42410.

TCP connections already support max_downstream_connection_duration_jitter_percentage (merged in #40686). This PR adds the equivalent feature for HTTP connections via a new max_connection_duration_jitter_percent field in HttpProtocolOptions.

Problem

When many HTTP/2 connections are established simultaneously (e.g. during a pod restart or rolling deploy), they all reach max_connection_duration at the same time, triggering simultaneous drain → reconnect storms ("thundering herd").

Solution

Add max_connection_duration_jitter_percent to HttpProtocolOptions. When configured, the effective max_connection_duration for each connection is individually extended by a random amount uniformly distributed in [0, jitter_percent/100 * base_duration], spreading connection drains over a window rather than synchronizing them.

Example: max_connection_duration = 10s, max_connection_duration_jitter_percent = 25 → each connection drains at a random time in [10s, 12.5s].

Changes

File	Change
api/envoy/config/core/v3/protocol.proto	Add max_connection_duration_jitter_percent (field 8) to HttpProtocolOptions
source/common/http/conn_manager_config.h	Add maxConnectionDurationJitterPercent() pure virtual method
source/extensions/filters/network/http_connection_manager/config.{h,cc}	Parse and store the new proto field
source/common/http/conn_manager_impl.cc	Apply jitter when arming the connection duration timer
source/server/admin/admin.h	Stub: returns absl::nullopt (no jitter for admin connections)
test/mocks/http/mocks.h	Add MOCK_METHOD for new interface method
test/common/http/conn_manager_impl_test_base.{h,cc}	Add stub forwarding delegates
test/common/http/conn_manager_impl_fuzz_test.cc	Add stub returning absl::nullopt

Prior art

The TCP proxy implementation in source/common/tcp_proxy/tcp_proxy.cc (method Config::calculateMaxDownstreamConnectionDurationWithJitter()) provides the exact same pattern — this PR follows it faithfully.

AI Disclosure: Used GitHub Copilot for coding assistance.

AI disclosure: GitHub Copilot was used during implementation and test writing. I fully understand all changes made in this PR.

Commit Message: See PR title
Risk Level: Low
Testing: Unit tests added/verified
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A

[repokitteh-read-only]

Hi @Retr0-XD, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #44064 was opened by Retr0-XD.

see: more, trace.

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @markdroth
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #44064 was opened by Retr0-XD.

see: more, trace.