Add README

crates/ruff_metrics/README.md

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+# Metrics
+
+red-knot can generate metrics that describe its performance while type-checking Python code. To
+activate metrics collection, pass the `--metrics` option when invoking red-knot:
+
+```console
+$ red_knot check --metrics [rest of arguments]
+```
+
+This will cause red-knot to _append_ metrics for the current run to a file called `metrics.json` in
+the current directory.
+
+You can then use the `plot_metrics.py` file to generate graphs of those metrics:
+
+```console
+$ uv run crates/ruff_metrics/plot_metrics.py counter semantic_index.scope_count --group-by file
+```
+
+## Available plots
+
+### `counter`
+
+Shows how the value of a counter increases over time. You can optionally group by one of the counter
+metric's fields. Times are shown relative to the start of the process.
+
+```console
+$ uv run crates/ruff_metrics/plot_metrics.py counter semantic_index.scope_count --group-by file
+```
+
+### `histogram`
+
+Shows the distribution of values of a counter. You must provide a metric field to group by; shows
+the maxmimum values of the counter for each value of this field.
+
+```console
+$ uv run crates/ruff_metrics/plot_metrics.py histogram semantic_index.scope_count --group-by file
+```
+
+## Overriding the metrics file
+
+You can optionally provide a filename for the `--metrics` option, in which case we will output
+metrics data to that file instead of `./metrics.json`:
+
+```console
+$ red_knot check --metrics some-other-file.json [rest of arguments]
+```
+
+You can then pass the same filename to the `plot_metrics.py` script:
+
+```console
+$ uv run crates/ruff_metrics/plot_metrics.py --metrics some-other-file.json counter semantic_index.scope_count --group-by file
+```
+
+(Note that the `--metrics` option must come before the subcommand selecting which kind of plot you
+want.)