Include Rails log_tags in metadata

Nobody has claimed this yet.

I've made this change locally. But I think it could go in the public gem. I'm using `ActiveSupport::TaggedLogging` in Rails to include log tags like this:

```ruby
# This is the default in Rails, but could include more stuff if desired
config.log_tags = [:request_id]

# Send logs to LogDNA
log_dna_logger = Logdna::Ruby.new(
Rails.application.secrets.log_dna_api_key, {
env: "development",
app: "ShayHi",
level: Logger::DEBUG
}
)
# Wrap LogDNA logger in TaggedLogging to add log tags
config.logger = ActiveSupport::TaggedLogging.new(log_dna_logger)
```

I then modified `Logdna::Ruby#log` to include the tags in the metadata

```ruby
def log(message = nil, opts = {})
# MY NEW CODE BELOW vvv
if formatter.current_tags.present?
opts[:meta] = (opts[:meta] || {}).merge(tags: formatter.current_tags.join(" "))
end
# MY NEW CODE ABOVE ^^^
if message.nil? && block_given?
message = yield
end
if message.nil?
@internal_logger.debug("provide either a message or block")
return
end
message = message.to_s.encode("UTF-8")
@client.write_to_buffer(message, default_opts.merge(opts).merge(
timestamp: (Time.now.to_f * 1000).to_i
))
end
```

Logs now look like this:

This is particularly useful for including the Rails request_id in the log metadata. I can open a PR if this is something you're interested in adding to the gem for everyone.

Start by locating Logdna::Ruby#log and the logger setup used with ActiveSupport::TaggedLogging. Verify how current tags should be added to log metadata while preserving existing metadata, then confirm that tagged Rails logs include the request_id in the resulting metadata.

Written by the indexing model from the issue text.