ExporterExporter improvements

[inqrphl]

Fix 'url prefix' not working. ExporterExporter was registering the prefixes correctly so that listener routed them to the ExporterExporter. But it was not removing the prefix in ServeHTTP before checking if the remaining portion is "/list" or "/proxy"

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Read modules: The ExporterExporter can specify a module configuration path, in which multiple module config files can be put inside. If opening/parsing/validating one of the configs failed, it would return an error that would propage upwards to the snclient module initialization system. This would make snclient module agent exit, due to ExporterExporter module not being able to start.

In this PR, it just skips over the modules that cause errors, this can be either due to bad YAML syntax, file being unparsable as a module config file, file not being readable etc.

Even if all of the module config files are wrong, it does not return an error and cause snclient to stop.

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File watcher: The module directory is now watched for changes using fsnotify. This uses a new library: fsnotify , which uses inodes on Linux and 'ReadDirectoryChangesW' on windows. Seems to be more robust than manually checking files.

If a file is written into, deleted or renamed an event is fired. The events that are related to non .yml / .yaml files are ignored. If so, the modules are regenerated from the module config folder. Until the regeneration is complete, the exporter_exporter uses the old parsed config, but once the new one is ready it is switched to new version. The change is protected with a mutex. After a change new events that come for 5 seconds are ignored in a debounce period.

I checked this both by hand in Linux and Windows, seems to be notificing and firing events, and reloading the modules index without problems.

curl http://127.0.0.1:[port]/[url prefix]/list -u :[password]

# prefix is set to 'hello' , password is set to 'nicepassword'
curl 127.0.0.1:8443/hello/list -u :nicepassword
<h2>Exporters:</h2>
<ul>
<li><a href="/hello/proxy?module=node-zart">node-zart</a></li>
<li><a href="/hello/proxy?module=node">node</a></li>
</ul>

the file watcher is toggleable using the 'modules dir watcher' option. By default it is false, so only users needing this may turn it on knowing its possible side effects.

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Add 'allowed methods' configuration point.

Exporter_exporter has multiple ways of getting results from a prometheus conforming exporter. Https does a query and gets the output, file reads from a file (which is presumably refreshed from time to time) and exec calls an exporter directly.

Add a whitelist in the exporter_exporter configuration section called 'allowed methods', if its populated it will reject module definitions that dont use its methods. Is meant to protect against 'exec' types of modules.

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[inqrphl]

@lgmu

I did implement the file watcher now, and it has a debounce period of 5 seconds.

The file watcher sends events through a channel. To not reload modules for every single event, I implemented a debounce mechanism. Meaning it ignores events for 5 seconds after a reload of modules was triggered. Is that ok?

Secondly, the watcher does not care if the event added/modified/deleted a file that ends with .yml or .yaml extension. I did this so that even writing to a "timestamp" file the current time can trigger a reload of the modules. Thought this would help out due script. I can either leave it out like that, or throw away events that dont work on files that arent YAML