Support OpenTelemetry Environment Variables

[wenkokke]

The eventlog-live-otelcol program should support part of the OpenTelemetry Environment Variables.

It should respect the following variables and their counterparts for METRICS and TRACES, as described in the (OpenTelemetry Protocol Exporter)[https://opentelemetry.io/docs/specs/otel/protocol/exporter/]:

• OTEL_EXPORTER_OTLP_ENDPOINT
• OTEL_EXPORTER_OTLP_INSECURE
• OTEL_EXPORTER_OTLP_CLIENT_KEY
• OTEL_EXPORTER_OTLP_CLIENT_CERTIFICATE
• OTEL_EXPORTER_OTLP_HEADERS (blocked on grapesy)
• OTEL_EXPORTER_OTLP_COMPRESSION
• OTEL_EXPORTER_OTLP_TIMEOUT
• OTEL_EXPORTER_OTLP_PROTOCOL (at least grpc)

It should respect the following variables as described in the General SDK Configuration:

• OTEL_SDK_DISABLED
• OTEL_ENTITIES
• OTEL_RESOURCE_ATTRIBUTES
• OTEL_SERVICE_NAME
• OTEL_LOG_LEVEL

There isn't much sense in supporting OTEL_SDK_DISABLED.
If this is set to true, there is no reason to run eventlog-live-otelcol

If #82 is implemented, that opens the possibility of respecting the export intervals:

• OTEL_METRIC_EXPORT_INTERVAL
• OTEL_METRIC_EXPORT_TIMEOUT

There isn't much sense in supporting the exporter specifications, as the program only supports OTLP. However, it may be wise to at least throw an error if an unsupported exporter is selected:

• OTEL_TRACES_EXPORTER
• OTEL_METRICS_EXPORTER

If #21 is implemented, that would automatically implement this issue. However, this is blocked on support for metrics in hs-opentelemetry. Hence, it may be valuable to implement this issue as a transitional measure.