@WebServlet
class ShoppingCartServlet extends AbstractHttpServlet {
    @Override
    void doGet(HttpServletRequest req, HttpServletResponse resp) {
        // Get the active span
        final Span span = GlobalTracer.get().activeSpan();
        if (span != null) {
          // customer_id -> 254889
          // customer_tier -> platinum
          // cart_value -> 867
          span.setTag("customer.id", customer_id);
          span.setTag("customer.tier", customer_tier);
          span.setTag("cart.value", cart_value);
        }
        // [...]
    }
}

@Getter
@Setter
@ConfigurationProperties(prefix = "custom.datadog")
public class CustomDatadogProperties {

	private List<String> includeUrlPatterns = new ArrayList<>();
	private List<String> targetFields = new ArrayList<>();
	private boolean enabled = false;
}

@Configuration
@ConditionalOnProperty(value = "custom.datadog.enabled", havingValue = "true")
@EnableConfigurationProperties(CustomDatadogProperties.class)
public class CustomDatadogPropertiesInitializer {

	private Map<String, String> targetFieldMap = new HashMap<>();

	@Getter
	private String podName = "";

	@Getter
	private List<String> includeUrlPatterns = new ArrayList<>();

	public CustomPromDatadogPropertiesInitializer(CustomDatadogProperties customDatadogProperties) {
		initialize(customDatadogProperties);
	}

	private void initialize(CustomDatadogProperties customDatadogProperties) {

		List<String> targetFields = customDatadogProperties.getTargetFields();

		targetFields.stream()
				.flatMap(fields -> Arrays.stream(fields.split(",\\s*")))
				.map(String::trim)
				.forEach(field -> targetFieldMap.put(field, convertToUnderscore(field)));

		includeUrlPatterns = customDatadogProperties.getIncludeUrlPatterns();

		try {
			String hostname = InetAddress.getLocalHost().getHostName();
			if (StringUtils.isNotBlank(hostname)) {
				podName = hostname;
			}
		} catch (UnknownHostException e) {
			podName = "알수없음";
		}
	}

	public String getTagName(String key) {
		return targetFieldMap.get(key);
	}

	private String convertToUnderscore(String str) {

		try {
			return str.replaceAll("([a-z])([A-Z])", "$1_$2").toLowerCase();
		} catch (Exception e) {
			return str;
		}
	}
}

@Configuration
@ConditionalOnBean(CustomDatadogPropertiesInitializer.class)
public class CustomDatadogAutoConfiguration {

	@Bean
	@DependsOn("customDatadogPropertiesInitializer")
	public FilterRegistrationBean<CustomDatadogFilter> customDatadogFilter(ObjectMapper objectMapper, CustomDatadogPropertiesInitializer customDatadogPropertiesInitializer) {

		List<String> includeUrlPatterns = customDatadogPropertiesInitializer.getIncludeUrlPatterns();
		FilterRegistrationBean<CustomDatadogFilter> registrationBean = new FilterRegistrationBean<>();

		registrationBean.setFilter(new CustomDatadogFilter(objectMapper, customDatadogPropertiesInitializer));

		if (!includeUrlPatterns.isEmpty()) {
			registrationBean.addUrlPatterns(includeUrlPatterns.toArray(new String[includeUrlPatterns.size()]));
		}

		registrationBean.setName("customDatadogFilter");
		return registrationBean;
	}
}

@Slf4j
@RequiredArgsConstructor
public class CustomDatadogFilter extends OncePerRequestFilter {

	private final ObjectMapper objectMapper;
	private final CustomDatadogPropertiesInitializer customDatadogPropertiesInitializer;
	private final DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");

	public static final String TAG_PREFIX = "params.";

	@Override
	protected void doFilterInternal(HttpServletRequest req, HttpServletResponse res, FilterChain filterChain) throws ServletException, IOException {

		ContentCachingRequestWrapper cacheReq = new ContentCachingRequestWrapper(req);
		ContentCachingResponseWrapper cacheRes = new ContentCachingResponseWrapper(res);

		filterChain.doFilter(cacheReq, cacheRes);

		Map<String, Object> paramMap = new HashMap<>();

		try {
			Map<String, Object> reqMap = parseRequestBody(cacheReq);
			Map<String, Object> resMap = parseResponseBody(cacheRes);

			paramMap.putAll(cacheReq.getParameterMap());
			paramMap.putAll(reqMap);
			paramMap.putAll(resMap);
		} catch (Exception e) {
			// parse 도중 에러가 나면 paramMap을 비워둔 채로 마무리 한다.
		}

		final Span span = GlobalTracer.get().activeSpan();

		if (span != null) {

			paramMap.forEach((key, value) -> {

				String tagName = customDatadogPropertiesInitializer.getTagName(key);
				if (StringUtils.isNotBlank(tagName)) {
					span.setTag(TAG_PREFIX + tagName, String.valueOf(value));
				}
			});

			span.setTag(TAG_PREFIX + "pod_name", customDatadogPropertiesInitializer.getPodName());

		}

		cacheRes.copyBodyToResponse();
	}

	private Map<String, Object> parseRequestBody(ContentCachingRequestWrapper cacheReq) {

		try {
			byte[] buf = cacheReq.getContentAsByteArray();

			if (buf.length > 0) {

				Map<String, Object> resultMap = new LinkedHashMap<>();
				Map<String, Object> resMap = objectMapper.readValue(buf, new TypeReference<Map<String, Object>>() {
				});

				resMap.entrySet().forEach(res -> {
					String key = res.getKey();
					Object value = res.getValue();

					resultMap.put(key, convertToString(value));
				});

				return resultMap;
			}
		} catch (Exception e) {
			log.error("CustomDatadogFilter parseRequestBody exception", e);
		}

		return Collections.emptyMap();
	}

	private Map<String, Object> parseResponseBody(ContentCachingResponseWrapper cacheRes) {

		try {
			byte[] buf = cacheRes.getContentAsByteArray();

			if (buf.length > 0) {

				Map<String, Object> resultMap = new LinkedHashMap<>();
				Map<String, Object> resMap = objectMapper.readValue(buf, new TypeReference<Map<String, Object>>() {
				});

				resMap.entrySet().forEach(res -> {
					String key = res.getKey();
					Object value = res.getValue();

					resultMap.put(key, convertToString(value));
				});

				return resultMap;
			}
		} catch (Exception e) {
			log.error("CustomDatadogFilter parseRequestBody exception", e);
		}

		return Collections.emptyMap();
	}

	private String convertToString(Object value) {

		try {
			if (value instanceof Date) {
				Date date = (Date) value;
				return formatter.format(date.toInstant().atZone(ZoneId.systemDefault()).toLocalDateTime());
			} else if (value instanceof Number || value instanceof String || value instanceof Boolean) {
				return String.valueOf(value);
			}
		} catch (Exception e) {
			log.error("CustomDatadogFilter convertToString exception", e);
		}

		return "";
	}
}