operationProfiling.filter config option rejects agg expressions

For example, this mongod.conf fails to parse:

operationProfiling.filter: "{$sampleRate: 0.01}"

"Failed to parse option operationProfiling.filter: Unrecognized expression '$lt'"

But it's a valid filter, and the profile command accepts it. It looks like we are parsing this option before the map of aggregation expression parsers is populated.