something like this if i understood correctly

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  
  <xsl:output indent="yes"/>
  <xsl:key name="id" match="service" use="id"/>
  
  <xsl:template match="/">
    <xsl:for-each select="/x/session/service[generate-id()=generate-id(key('id',id))]">
      <service>
	<xsl:copy-of select="name|id"/>
	<xsl:for-each select="key('id',id)/../metric">
	  <xsl:sort select="@name"/>
	  <xsl:copy-of select="."/>
	</xsl:for-each>
      </service>
    </xsl:for-each>
    </xsl:template>
</xsl:stylesheet>

$ saxon session.xml session.xsl
<?xml version="1.0" encoding="utf-8"?>
<service>
   <name>somename</name>
   <id>1</id>
   <metric name="andanothermetric">value3</metric>
   <metric name="andanothermetric">value6</metric>
   <metric name="anothermetric">value2</metric>
   <metric name="anothermetric">value5</metric>
   <metric name="somemetric">value1</metric>
   <metric name="somemetric">value4</metric>
</service>
<service>
   <name>anothername</name>
   <id>2</id>
   <metric name="andanothermetric">value9</metric>
   <metric name="andanothermetric">value12</metric>
   <metric name="anothermetric">value8</metric>
   <metric name="anothermetric">value11</metric>
   <metric name="somemetric">value7</metric>
   <metric name="somemetric">value10</metric>
</service>