add splunk_otlp_histograms resource attr

[jinja2]

Description:

Adds the resource attribute splunk_otlp_histograms: true to the service::telemetry::resource if the collector detects any in use signalfx metrics exporter with the flag send_otlp_histograms enabled.

Link to Splunk idea: <Link to Splunk idea, see https://ideas.splunk.com>

Testing:
Added tests

Documentation:

[theletterf]

@jinja2 From an end-user perspective, enabling histograms in SFx exporter would still be the same? Is this transparent to the end users configuring the Collector for histogram export?

[jinja2]

@theletterf yes, this change is transparent to the user. We want to use this attribute to track usage of the feature on our end.