Add invalid metric names to performance insights

Author: ckbedwell

src/data/markdown/docs/03 cloud/02 Analyzing Results/02 Performance Insights.md

@@ -25,6 +25,7 @@ k6 categorizes performance insights into three sets:
 - [Too many metrics](#too-many-metrics)
 - [Too many time series](#too-many-time-series)
 - [Outdated k6 Release Used](#outdated-k6-release-used)
+- [Invalid Metric Names](#invalid-metric-names)
 - [High load generator CPU usage](#high-load-generator-cpu-usage)
 - [High load generator memory usage](#high-load-generator-memory-usage)
 
@@ -248,6 +249,17 @@ for (let id = 1; id <= 1000; id++) {
   - Update the k6 binary that your CI/CD pipeline uses to run tests.
   - If you're part of an organization or team, collectively decide on a version of k6 to use going forward for consistency and ease of comparison.
 
+### Invalid Metric Names
+
+_Identifier_: `best_practice_invalid_metric_names`
+
+- **Happens when**:
+  you tag your custom metrics with invalid names.
+- **Recommendations**:
+  - Ensure your custom metric names comply with OpenTelemetry and Prometheus limitations. Such as:
+    - they only contain valid symbols of ASCII letters, numbers, or `_`.
+    - Metric names must not start with a number.
+
 ## Health alerts
 
 Health alerts happen when the load generator has high resource utilization.
@@ -367,6 +379,7 @@ For all insights and their identifiers, refer to the table below:
 | Too Many Metrics                 | `best_practice_too_many_metrics`         | `best_practice` |
 | Too Many Time Series             | `best_practice_too_many_time_series`     | `best_practice` |
 | Outdated k6 Release Used         | `best_practice_outdated_k6_release_used` | `best_practice` |
+| Invalid Metric Names             | `best_practice_invalid_metric_names    ` | `best_practice` |
 | High Load Generator CPU Usage    | `health_high_loadgen_cpu_usage`          | `health`        |
 | High Load Generator Memory Usage | `health_high_loadgen_mem_usage`          | `health`        |