Documentation: create doc describing grpc-go's log levels and their usages

Documentation/log_levels.md

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+# Log Levels
+
+This document describes the different log levels supported by the grpc-go
+library, and under what conditions they should be used.
+
+### Info
+
+Info messages are for informational purposes and may aid in the debugging of
+applications or the gRPC library.
+
+Examples:
+- The name resolver received an update.
+- The balancer updated its picker.
+- Significant gRPC state is changing.
+
+At verbosity of 0 (the default), any single info message should not be output
+more than once every 5 minutes under normal operation.
+
+### Warning
+
+Warning messages indicate problems that are non-fatal for the application, but
+could lead to unexpected behavior or subsequent errors.
+
+Examples:
+- Resolver could not resolve target name.
+- Error received while connecting to a server.
+- Lost or corrupt connection with remote endpoint.
+
+### Error
+
+Error messages represent errors in the usage of gRPC that cannot be returned to
+the application as errors, or internal gRPC-Go errors that are recoverable.
+
+Internal errors are detected during gRPC tests and will result in test failures.
+
+Examples:
+- Invalid arguments passed to a function that cannot return an error.
+- An internal error that cannot be returned or would be inappropriate to return
+  to the user.
+
+### Fatal
+
+Fatal errors are severe internal errors that are unrecoverable.  These lead
+directly to panics, and are avoided as much as possible.
+
+Example:
+- Internal invariant was violated.
+- User attempted an action that cannot return an error gracefully, but would
+  lead to an invalid state if performed.