Update OpenTelemetry_OPEA_Guide.rst

Author: louie-tsai

tutorial/OpenTelemetry/OpenTelemetry_OPEA_Guide.rst

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 .. _OpenTelemetry_OPEA_Guide:
 
 OpenTelemetry on OPEA Gudie
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 Overview
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    ChatQnA <deploy/ChatQnA>
 
 How to Monitor 
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 OpenTelemetry metrics and tracing can be visualized through three primary monitoring UI web pages.  
 
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 Code Instrumentations for OPEA Tracing
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 Enabling OPEA OpenTelemetry tracing for a function is straightforward. 
 First, import opea_telemetry, and then apply the Python decorator @opea_telemetry to the function you wish to trace.