fix(trace): Hide span-only metrics from trace waterfall tab

[nsdeschenes]

Summary

• exclude span-sourced metrics from trace-details metric availability and counts
• use the filtered count for tab visibility, header metadata, empty traces, and project discovery
• keep span-sourced metrics out of the Application Metrics samples and autocomplete queries
• preserve metrics with no source attribute or any source other than span

Closes EXP-1153

[linear-code]

EXP-1153

[cursor]

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