feat(inputs.prometheus): use system wide proxy settings

[erwiese]

resolves #11728

[powersj]

Hi,

Thanks for taking the time to put up a PR.

In Telegraf, we already have a standard HTTP client config that allows users to set the proxy as a part of their configuration. It would be good to use the same behavior across plugins.

Take a look at the HTTP plugin in how it creates a client with the additional httpconfig.HTTPClientConfig.

As well, we would to add the configuration options for the proxy to the plugin configuration.

[erwiese]

Hi, could you already review my changes? or anything else missing?

[srebhan]

@erwiese, your change looks a lot better this way. Thanks for updating the PR! I have one request left to avoid recreating the client over and over. Can you please take a look?

[Hipska]

Shouldn't the unix socket client stuff also be added to Init in this case? It is also making a new client on each interval + URL. Both (http & unix) can be placed in the same p.client. Furthermore the response_timeout can be deprecated in favour of timeout.

Edit: be careful, It seems there is possibility of mixed types of URLs, some can be http and/or unix..

[erwiese]

@Hipska someone could use unix and http URLs in one call, so I will not touch it. response_time: yes but breaking and not matter of this issue.

[Hipska]

response_time could be marked as deprecated and still being used when timeout is not set..

[telegraf-tiger]

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DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_i386.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz
		static_linux_amd64.tar.gz