manual-instr: add footer

Signed-off-by: Benedikt Bongartz <bongartz@klimlive.de>

Author: frzifus

04-manual-instrumentation.md

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 +        image: ghcr.io/frzifus/kubecon-eu-2024-opentelemetry-kubernetes-tracing-tutorial-backend4:with-instr
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+By instrumenting our applications, whether manually or automatically, we get more telemetry data to help us understand our system. However, since a large amount of telemetry data also generates costs, in the next chapter we will discuss how we can utilise this amount in a meaningful way. 
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 [Next steps](./05-sampling.md)