Diff configs script (#8944)

**What this PR does / why we need it**:

I always forget this trick for diffing configs when preparing a new
release so I added it to a script.

tools/diff-config.sh

@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+
+current_dir="$(cd "$(dirname "${0}")" && pwd)"
+loki_dir="$(cd "${current_dir}/../cmd/loki" && pwd)"
+
+export OLD_LOKI=${OLD_VERSION:-2.7.5}
+export NEW_LOKI=${NEW_VERSION:-$("${current_dir}/image-tag")}
+
+export CONFIG_FILE="loki-local-config.yaml"
+
+function get_config() {
+	local version=${1}
+
+	docker run --rm -t -v "${loki_dir}:/config" "grafana/loki:${version}" \
+		-config.file="/config/${CONFIG_FILE}" \
+		-print-config-stderr 2>&1
+}
+
+function parse_config() {
+	sed '/Starting Loki/q' | tr -d '\r'
+}
+
+tmp_dir="$(mktemp -d)"
+old_config="${tmp_dir}/config-${OLD_LOKI}.yml"
+new_config="${tmp_dir}/config-${NEW_LOKI}.yml"
+
+echo "Saving configs to ${tmp_dir}"
+echo "Old config: ${old_config}"
+echo "New config: ${new_config}"
+
+get_config "${OLD_LOKI}" | parse_config > "${old_config}"
+get_config "${NEW_LOKI}" | parse_config > "${new_config}"
+
+diff --color=always \
+  --side-by-side \
+  "${old_config}" \
+  "${new_config}"