Permit comma-separated list of module names in "module" parameter

[candlerb]

This is a proof-of-concept patch for discussion and review.

It allows a single scrape to specify a comma-separated list of modules, e.g. module=if_mib,mikrotik. This implements #731, and supports the breaking up of snmp.yml into independent MIB pieces as discussed in #938.

Points to note/discuss (some marked TODO in source):

• Currently only module=foo,bar is supported. I could add module=foo&module=bar as an additional option; however, I want to keep the former, because target rewriting rules can only specify a single instance of a given parameter (i.e. __param_module in this case)
• There's no attempt to reject duplicate modules (module=if_mib,if_mib), nor to handle distinct modules which generate overlapping metrics. In those cases, the Prometheus client library itself will barf, with an error like

  * collected metric "sysUpTime" { gauge:{value:7.102146e+08}} was collected before with the same name and label values
  

• The modules are processed sequentially, in the order given in the request
• Each module still has a scrape phase and an emit-metrics phase. The scrape times (and packet count / retry count) are summed, to give the same metrics
• I have not added any additional tests - but none of the existing ones break. I believe that if you invoke this with a single module, it should behave exactly the same as today.
• I have not updated documentation

[SuperQ]

We're already working on multiple modules in #945.

[candlerb]

Closing in favour of #945 (but will leave the branch just in case)