Update OPENTSDB_PYTHON_METRICS_TEST_MODE description

Reflect the change in https://github.com/LCOGT/opentsdb_python_metrics which now correctly parses false values in `OPENTSDB_PYTHON_METRICS_TEST_MODE`. Previously, any value was evaluated as `True`.

Author: timbeccue

README.md

@@ -49,7 +49,7 @@ set as environment variables.
 |                 | `AWS_DEFAULT_REGION`                | AWS S3 Default Region                                                                                                                                                                                                                      | _empty string_             |
 |                 | `S3_ENDPOINT_URL`                   | Endpoint url for connecting to s3. This can be modified to connect to a local instance of s3.                                                                                                                                              | `"http://s3.us-west-2.amazonaws.com"` |
 | Metrics         | `OPENTSDB_HOSTNAME`                 | OpenTSDB Host to send metrics to                                                                                                                                                                                                           | _empty string_             |
-|                 | `OPENTSDB_PYTHON_METRICS_TEST_MODE` | Set to any value to turn off metrics collection                                                                                                                                                                                            | `False`                    |
+|                 | `OPENTSDB_PYTHON_METRICS_TEST_MODE` | Test mode turns off metrics collection. `True` or `1` to turn on test mode; `False` or `0` or unset to keep test mode off.                                                                                                                 | `False`                    |
 |                 | `INGESTER_PROCESS_NAME`             | A tag set with the collected metrics to identify where the metrics are coming from                                                                                                                                                         | `ingester`                 |
 |                 | `SUBMIT_METRICS_ASYNCHRONOUSLY`     | Optionally submit metrics asynchronously. This option does not apply when the command line entrypoint is used, in which case metrics are always submitted synchronously. Note that some metrics may be lost when submitted asynchronously. | `False`                    |