Do not set appname if set in URI

[kevinAlbs]

Summary Only call mongoc_client_set_appname if the appname URI option is not set.

Motivation

appname is a URI option users may set themselves: https://docs.mongodb.com/manual/reference/connection-string/#miscellaneous-configuration

Before this change:

Attempting to set the appname in the URI results in a warning:

> m <- mongo(url="mongodb://localhost:27017/?appname=foo")
Warning: [ERROR] Cannot set appname more than once

The appname set in the URI is applied first. Observing mongod logs of the handshake shows application.name is foo.

{"t":{"$date":"2021-08-28T14:51:26.131-04:00"},"s":"I",  "c":"NETWORK",  "id":51800,   "ctx":"conn8307","msg":"client metadata","attr":{"remote":"127.0.0.1:60929","client":"conn8307","doc":{"application":{"name":"foo"},"driver":{"name":"mongoc","version":"1.16.2"},"os":{"type":"Darwin","name":"macOS","version":"20.5.0","architecture":"x86_64"},"platform":"cfg=0x000216aa65 posix=200112 stdc=201710 CC=clang 12.0.5 (clang-1205.0.22.11) CFLAGS=\"\" LDFLAGS=\"\""}}}

After this change:

No warning is reported by mongolite.

Rejected Alternatives

Alternativly, the call to mongoc_client_set_appname (client, "r/mongolite"); could be removed entirely since appname is not intended to be set by a driver.

However, removing the call could cause friction on users that are tracking mongolite clients through the existing r/mongolite appname.