[internal/splunk] Splunk metric names should be validated via regex

[spiffyy99]

Description: For the splunk HEC receiver, add additional criteria for the metric names. As per https://docs.splunk.com/Documentation/Splunk/9.2.1/Metrics/Overview#What_is_a_metric_data_point.3F, metric names can only consist of specific characters. Validates these requirements to be in line with other Splunk receivers.

Link to tracking Issue: #34275

Testing: Unit tests

Documentation: N/A

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: spiffyy99 / name: Ryan Min (b2fa86c, b6199a2, b9d9579)
• ✅ login: dmitryax / name: Dmitrii Anoshin (749e3b4)