[8.x](backport #39405) [metricbeat] [helper] Fix http server helper SSL config

[mergify]

Overview

The TLS config used in the metricbeat http server module uses a TLS config suitable for clients (BuildModuleClientConfig), not for servers. This makes it impossible to successfully connect to Prometheus remote_write using TLS since bad certificate is always returned.

Made some small unrelated changes to make golangci-lint happy. Added a reasonable ReadHeaderTimeout of 10 seconds. Replaced ioutil with io.

This was tested with openssl s_client and also directly with the remote_write data stream.

---
Post-Handshake New Session Ticket arrived:
SSL-Session:
Protocol : TLSv1.3
Cipher : TLS_AES_128_GCM_SHA256
Session-ID: 39C678F0ED9BBBFD657458330B59C50D3A9461C06B7727F5AAF667882F41BE5D
Session-ID-ctx:
Resumption PSK: 5C4509D103373B8036FE089AD2F79BFDC411A2B5C1439CE5852AF8F1184A9EA5
PSK identity: None
PSK identity hint: None
SRP username: None
TLS session ticket lifetime hint: 604800 (seconds)
TLS session ticket:
0000 - d6 b1 2b 3f f0 69 ea 9a-68 e1 0c e4 24 87 e3 ec ..+?.i..h...$...
0010 - 64 ff 69 a4 4d 2e 88 52-03 64 52 63 91 bc 30 04 d.i.M..R.dRc..0.
0020 - 49 55 1d 1a 79 6c 2b 86-54 46 ae c0 58 95 07 d7 IU..yl+.TF..X...
0030 - c3 b4 f4 83 8c 76 c7 8a-19 5b f6 da 8a b7 55 7f .....v...[....U.
0040 - 42 52 e2 71 17 f1 9a 76-3a f3 1e 29 b5 6a e4 1e BR.q...v:..).j..
0050 - b7 ff 89 4d ef 34 ba 6d-9f 9f df 7a 86 96 c3 59 ...M.4.m...z...Y
0060 - 53 81 5a 17 c2 f6 7f a7-55 4d b1 94 d8 0f a7 69 S.Z.....UM.....i
0070 - c3 .

Start Time: 1712569088
Timeout : 7200 (sec)
Verify return code: 0 (ok)
Extended master secret: no
Max Early Data: 0
---
read R BLOCK
800B0789E27F0000:error:0A000412:SSL routines:ssl3_read_bytes:sslv3 alert bad certificate:ssl/record/rec_layer_s3.c:1600:SSL alert number 42

The NewHttpServer is also used by the http module.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Relates [prometheus] [remote_write] SSL bad certificate integrations#9697

Use cases

Screenshots

Logs

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This is an automatic backport of pull request #39405 done by [Mergify](https://mergify.com).

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