Added basic opentelemetry docs

Co-authored-by: Peter Rifel <[email protected]>

docs/opentelemetry.md

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+# OpenTelemetry support
+
+kOps is experimenting with initial support for OpenTelemetry, starting with tracing.
+
+The support should be considered experimental; the trace file format and schema will likely change, and these initial experiments might be removed entirely.
+
+kOps supports a "serverless" mode of operation, where it logs the OpenTracing output to a file.  We do this because our e2e test runner (prow) doesn't yet have a destination for OpenTelemetry data.
+
+To try this out:
+
+`OTEL_EXPORTER_OTLP_TRACES_FILE=/tmp/trace go run ./cmd/kops get cluster`
+
+You should now see that the /tmp/trace file is created.
+
+Then we have an experimental tool to serve the trace file to jaeger:
+
+```
+cd tools/otel/traceserver
+go run . --src /tmp/trace --run jaeger
+```
+
+Not everything is instrumented yet, and not all the traces are fully joined up (we need to thread more contexts through more methods),
+but you should be able to start to explore the operations that we run and their performance.