Support configuration files

[pavolloffay]

Support configuration files. This allows us to configure Jaeger with kubernetes config maps.

By default configuration files are disabled. Lists are currently not handled because we parse them manually. For instance cassandra.servers requires a string with comma separated values.

Follows some examples. Note that flags with dots can be written as nested objects.

{
  "query": {
    "prefix": "prefix",
    "static-files": "static"
  },
  "cassandra": {
    "keyspace": "from_file",
    "password": "pas_from_file",
    "servers": "localhost,localhost2",
    "archive": {
      "keyspace": "archive_keyspace"
    }
  },
  "es.archive.max-span-age": "75h0m0s"
}

query:
  prefix: prefix
  static-files: static
cassandra:
  keyspace: from_file
  password: pas_from_file
  servers: localhost,localhost2
  archive:
    keyspace: archive_keyspace
es.archive.max-span-age: 75h0m0s

[coveralls]

Coverage remained the same at 100.0% when pulling 55718a7 on pavolloffay:viper-config-files into d17fad0 on jaegertracing:master.

[yurishkuro]

overall looks like a great feature, but it looks like it could've been done with a lot less duplication by making it the default behavior of the SharedFlags

[yurishkuro]

any reason not to have config flag added as part of AddFlags? It would probably also avoid duplicating the code in L65-71 in all binaries, only do it in sFlags := new(flags.SharedFlags).InitFromViper(v)

[pavolloffay]

I did a split for two reasons:

1. agent does use flags.AddFlags
2. flags.AddFlags can be specified in config file, so there would be twice sFlags := new(flags.SharedFlags).InitFromViper(v)

[yurishkuro]

I see - makes sense. In that case can we move L65-71 into that package too, so that we can have just a one liner in the mains? E.g. something like flags.TryLoadConfigFile(v, logger)

[yurishkuro]

also, by doing that we can make flags.ExternalConfFlags type private. I suggest renaming it to ConfigFile(Flag) ("external" is non-informative)

deduplicate

[coveralls]

Coverage remained the same at 100.0% when pulling 912b81e on pavolloffay:viper-config-files into d17fad0 on jaegertracing:master.

[yurishkuro]

Nit: don't use string formating. File name can be passed as a string field. Message could be "error loading config".

[yurishkuro]

Nit: s/conf/config for making consistency

[coveralls]

Coverage remained the same at 100.0% when pulling 20697b1 on pavolloffay:viper-config-files into d17fad0 on jaegertracing:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 5c23082 on pavolloffay:viper-config-files into d17fad0 on jaegertracing:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 5891eb2 on pavolloffay:viper-config-files into 54151f8 on jaegertracing:master.