Add support for configuring HTTP Proxy

The previous behavior was to adopt the global proxy setting through
the use of the Net::HTTP module.  This change introduces two new
configuration parameters that allow for manually specifying the HTTP
Proxy to use for the InfluxDB connection.

This allows for circumstances where it may be required to use a
specific proxy only for the InfluxDB connection.

Author: epchris

README.md

@@ -94,6 +94,21 @@ url = "https://influxdb.example.com:8086/database_name?open_timeout=3"
 influxdb = InfluxDB::Client.new url: url, open_timeout: 10
 ```
 
+#### Using a custom HTTP Proxy
+
+By default, the `Net::HTTP` proxy behavior is used (see [Net::HTTP Proxy][proxy])
+You can optionally set a proxy address and port via the `proxy_addr` and
+`proxy_port` options:
+
+``` ruby
+influxdb = InfluxDB::Client.new database,
+  host:       "influxdb.domain.com",
+  proxy_addr: "your.proxy.addr",
+  proxy_port: 8080
+```
+
+[proxy]: https://docs.ruby-lang.org/en/2.7.0/Net/HTTP.html#class-Net::HTTP-label-Proxies
+
 ### Writing data
 
 Write some data:

lib/influxdb/client/http.rb

@@ -45,7 +45,7 @@ def connect_with_retry
       retry_count = 0
 
       begin
-        http = Net::HTTP.new(host, config.port)
+        http = build_http(host, config.port)
         http.open_timeout = config.open_timeout
         http.read_timeout = config.read_timeout
 
@@ -133,6 +133,16 @@ def generate_cert_store
       end
       store
     end
+
+    # Builds an http instance, taking into account any configured
+    # proxy configuration
+    def build_http(host, port)
+      if config.proxy_addr
+        Net::HTTP.new(host, port, config.proxy_addr, config.proxy_port)
+      else
+        Net::HTTP.new(host, port)
+      end
+    end
   end
   # rubocop:enable Metrics/MethodLength
   # rubocop:enable Metrics/AbcSize

lib/influxdb/config.rb

@@ -16,6 +16,8 @@ module InfluxDB
     open_timeout:         5,
     read_timeout:         300,
     auth_method:          nil,
+    proxy_addr:           nil,
+    proxy_port:           nil,
 
     # SSL options
     use_ssl:              false,

spec/influxdb/config_spec.rb

@@ -20,6 +20,8 @@
     specify { expect(conf).not_to be_udp }
     specify { expect(conf).not_to be_async }
     specify { expect(conf.epoch).to be_falsey }
+    specify { expect(conf.proxy_addr).to be_nil }
+    specify { expect(conf.proxy_port).to be_nil }
   end
 
   context "with no database specified" do
@@ -205,4 +207,21 @@
       expect(conf).not_to be_async
     end
   end
+
+  context "given explicit proxy information" do
+    let(:args) do
+      [{
+        host:           "host",
+        port:           "port",
+        username:       "username",
+        password:       "password",
+        time_precision: "m",
+        proxy_addr:     "my.proxy.addr",
+        proxy_port:     8080
+      }]
+    end
+
+    specify { expect(conf.proxy_addr).to eq("my.proxy.addr") }
+    specify { expect(conf.proxy_port).to eq(8080) }
+  end
 end