config: add "connection_limit" option to yaml config (#443)

This adds the `connection_limit` configuration option to the yaml
configuration `apm_config` section to match the `ini` settings.

config/merge_yaml.go

@@ -47,6 +47,7 @@ type traceAgent struct {
 	LogFilePath        string         `yaml:"log_file"`
 	ReplaceTags        []*ReplaceRule `yaml:"replace_tags"`
 	ReceiverPort       int            `yaml:"receiver_port"`
+	ConnectionLimit    int            `yaml:"connection_limit"`
 	APMNonLocalTraffic *bool          `yaml:"apm_non_local_traffic"`
 
 	WatchdogMaxMemory float64 `yaml:"max_memory"`
@@ -180,6 +181,10 @@ func (c *AgentConfig) loadYamlConfig(yc *YamlAgentConfig) {
 		c.ReceiverPort = yc.TraceAgent.ReceiverPort
 	}
 
+	if yc.TraceAgent.ConnectionLimit > 0 {
+		c.ConnectionLimit = yc.TraceAgent.ConnectionLimit
+	}
+
 	if yc.TraceAgent.ExtraSampleRate > 0 {
 		c.ExtraSampleRate = yc.TraceAgent.ExtraSampleRate
 	}