Introduce option to set TOS field in ICMP and other probes

[s1sfa]

I use the TOS field for PBR routing to monitor ping latency through different paths on my network.

Currently I use smokeping for this and it would be interesting to replace with this. Probe setup Smokeping config:

+ FPing
binary = /usr/bin/fping

++ FPingPath1
tos = 0x30

++ FPingPath2
tos = 0x20

++ FPingPath3
tos = 0x28

Or using the ping -Q flag ping 8.8.8.8 -Q 32

I've seen use cases in other tools where others like to set TOS for other tests not only for PBR but maybe want to set a certain queuing priority on their network for testing traffic which is why I think this feature can apply to all modules.

[brian-brazil]

Does Go offer a way to set this on things like http requests?

[fredericve]

I am not at all acquainted with Go, but it looks like it should be possible with Go 1.11 according to this post on SO

[electron0zero]

~Hi 👋🏼, coming from the cleanup 🧹~

~This PR is quite old and there's https://github.com/prometheus/blackbox_exporter/pull/613 looks to be superset of this PR.~

~I am going to close this, please feel free to reopen if you want to update it, and discuss it further.~

[electron0zero]

oops, meant to close the PR. reopening this issue.