Add support for a minimum TTL for events

When running with riemann-wrappers, one may want to run different tools
with distinct ttl, but the `--ttl` flag cannot be specified twice, so
the following configuration will not work:

```
---
options: --ttl 300
tools:
- name: "health"
- name: "rdap"
  options: "--interval 21600 --ttl 86400 --domains example.com example.net"
```

A workaround is to add an explicit ttl for each tool, but it is not
convenient.

Introduce a `--minimum-ttl` flag that can be used as a global option to
circumvent this issue.

lib/riemann/tools.rb

@@ -32,7 +32,8 @@ def options
         opt :event_host, 'Event hostname', type: String
         opt :interval, 'Seconds between updates', default: 5
         opt :tag, 'Tag to add to events', type: String, multi: true
-        opt :ttl, 'TTL for events', type: Integer
+        opt :ttl, 'TTL for events (twice the interval when unspecified)', type: Integer
+        opt :minimum_ttl, 'Minimum TTL for events', type: Integer, short: :none
         opt :attribute, 'Attribute to add to the event', type: String, multi: true
         opt :timeout, 'Timeout (in seconds) when waiting for acknowledgements', default: 30
         opt :tcp, 'Use TCP transport instead of UDP (improves reliability, slight overhead.', default: true
@@ -50,6 +51,9 @@ def initialize(allow_arguments: false)
       options
       @argv = ARGV.dup
       abort "Error: stray arguments: #{ARGV.map(&:inspect).join(', ')}" if ARGV.any? && !allow_arguments
+
+      options[:ttl] ||= options[:interval] * 2
+      options[:ttl] = [options[:minimum_ttl], options[:ttl]].compact.max
     end
 
     # Returns parsed options (cached) from command line.
@@ -68,7 +72,7 @@ def attributes
     def report(event)
       event[:tags] = event.fetch(:tags, []) + options[:tag]
 
-      event[:ttl] ||= options[:ttl] || (options[:interval] * 2)
+      event[:ttl] ||= options[:ttl]
 
       event[:host] = options[:event_host].dup if options[:event_host]
 

spec/riemann/tools_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require 'riemann/tools/health'
+
+class ExampleTool
+  include Riemann::Tools
+end
+
+RSpec.describe Riemann::Tools do
+  describe 'ttl' do
+    subject(:tool) { ExampleTool.new }
+
+    {
+      ''                           => 10,
+      '--ttl=60'                   => 60,
+      '--interval 10'              => 20,
+      '--minimum-ttl 300'          => 300,
+      '--minimum-ttl 300 --ttl=60' => 300,
+      '--minimum-ttl 30 --ttl 60'  => 60,
+    }.each do |argv, expected_ttl|
+      context "with #{argv}" do
+        before do
+          ARGV.replace argv.split
+        end
+
+        it { expect(tool.options[:ttl]).to eq expected_ttl }
+      end
+    end
+  end
+end