mapping dynamic xml nodes to static nodes - xslt

Question

I am looking for someone to assist in solving a seemingly simple problem.
I want to map a node of /fields[x]/message_id to a static node /MessageID0x for 5 entries in a list.
The source node is optional and may not exist.
The schema is below
I am just not seeing the obvious, I hope.
The source is defined as:
<xs:element name="fields">
<xs:complexType>
<xs:sequence>
<xs:element name="tenant_id" type="xs:normalizedString" minOccurs="0"/>
<xs:element name="message_id" type="xs:normalizedString" minOccurs="0"/>
Target is defined as:
<xs:element name="MessageID01" type="xs:normalizedString" minOccurs="0"/>
<xs:element name="MessageID02" type="xs:normalizedString" minOccurs="0"/>
<xs:element name="MessageID03" type="xs:normalizedString" minOccurs="0"/>
<xs:element name="MessageID04" type="xs:normalizedString" minOccurs="0"/>
<xs:element name="MessageID05" type="xs:normalizedString" minOccurs="0"/>
=== FROM ===========
<root>
<ID>2019Nov12_17</ID>
<PingResult>OK</PingResult>
<StartDateTime>2019-11-12T16:16:01</StartDateTime>
<EndDateTime>2019-11-12T17:16:01.771Z</EndDateTime>
<start>0</start>
<numFound>1</numFound>
<fields>
<tenant_id>KOCHIND_AX2</tenant_id>
<message_id>lid://infor.landmark.lmrkmt/15d8f834-7680-541e-0000-001d5dae3e7b</message_id>
</fields>
<fields>
<tenant_id>KOCHIND_AX2</tenant_id>
<message_id>lid://infor.landmark.lmrkmt/0535a86a-7680-1868-0000-07625db833c1</message_id>
</fields>
<fields>
<tenant_id>KOCHIND_AX2</tenant_id>
<message_id>lid://infor.landmark.lmrkmt/0535a86a-7680-1864-0000-03445db849c8</message_id>
</fields>
<fields>
<tenant_id>KOCHIND_AX2</tenant_id>
<message_id>lid://infor.landmark.lmrkmt/0535a86a-7680-1867-0000-01151db125c8</message_id>
</fields>
</root>
TO ===================
<root>
<ID>2019Nov12_17</ID>
<PingResult>OK</PingResult>
<StartDateTime>2019-11-12T16:16:01</StartDateTime>
<EndDateTime>2019-11-12T17:16:01.771Z</EndDateTime>
<start>0</start>
<numFound>1</numFound>
<MessageID01>lid://infor.landmark.lmrkmt/15d8f834-7680-541e-0000-001d5dae3e7b</MessageID01>
<MessageID02>lid://infor.landmark.lmrkmt/0535a86a-7680-1868-0000-07625db833c1</MessageID02>
<MessageID03>lid://infor.landmark.lmrkmt/0535a86a-7680-1868-0000-07625db833c8</MessageID03>
<MessageID04>lid://infor.landmark.lmrkmt/15d8f834-1864—3322-0000-03445db125c8</MessageID04>
<MessageID05>lid://infor.landmark.lmrkmt/15d8f834-7680-1867-0000-01151db125g4</MessageID05>
</root>

Answer 1

I believe the requested result can be achieved rather simply by doing:
XSLT 1.0
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:strip-space elements="*"/>
<!-- identity transform -->
<xsl:template match="#*|node()">
<xsl:copy>
<xsl:apply-templates select="#*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="fields">
<xsl:variable name="n">
<xsl:number format="01"/>
</xsl:variable>
<xsl:element name="MessageID{$n}">
<xsl:value-of select="message_id"/>
</xsl:element>
</xsl:template>
</xsl:stylesheet>
The result of transforming the example input will be:
<?xml version=\"1.0\" encoding=\"UTF-8\"?>
<root>
<ID>2019Nov12_17</ID>
<PingResult>OK</PingResult>
<StartDateTime>2019-11-12T16:16:01</StartDateTime>
<EndDateTime>2019-11-12T17:16:01.771Z</EndDateTime>
<start>0</start>
<numFound>1</numFound>
<MessageID01>lid://infor.landmark.lmrkmt/15d8f834-7680-541e-0000-001d5dae3e7b</MessageID01>
<MessageID02>lid://infor.landmark.lmrkmt/0535a86a-7680-1868-0000-07625db833c1</MessageID02>
<MessageID03>lid://infor.landmark.lmrkmt/0535a86a-7680-1864-0000-03445db849c8</MessageID03>
<MessageID04>lid://infor.landmark.lmrkmt/0535a86a-7680-1867-0000-01151db125c8</MessageID04>
</root>
which is different from the result you show - nevertheless, I suspect it is the correct one.
Do note that numbering sibling nodes by name is bad practice. It makes subsequent transformations much more difficult. If you need a number (although I don't see why you should), use an attribute.