Management API to stream agent logs

[Joe Kralicky]

We should implement a simple management API to allow viewing logs for any connected agent. An API should look something like this:

package control;
service Log {
  // Streams logs from the agent according to the parameters specified in the request.
  // The stream will be closed when the newest requested log is sent, unless 'follow'
  // is true, in which case the stream will remain open indefinitely and continue to
  // send logs in real time as they are generated.
  rpc StreamLogs(LogStreamRequest) returns (stream StructuredLogRecord);
}

message LogStreamRequest {
  google.protobuf.Timestamp since   = 1;
  google.protobuf.Timestamp until   = 2;
  repeated LogStreamFilter  filters = 3;
  bool                      follow  = 4;
}

message LogStreamFilter {
  // A glob pattern to match against the logger name. For example, "plugin.*"
  // will match all plugin loggers.
  optional string namePattern = 1;
  // Minimum log severity level to include in the stream.
  optional int32 level = 2;
}

// Represents a single structured log message. Roughly analagous to slog.Record.
message StructuredLogRecord {
  google.protobuf.Timestamp time       = 1;
  string                    message    = 2;
  repeated Attr             attributes = 3;
}

// A key-value pair analagous to slog.Attr.
message Attr {
  string              key   = 1;
  google.protobuf.Any value = 2;
}